[cxx-interop] Make test discovery compatible with C++ interop (#7165)

### Motivation:

If a Swift module is built with C++ interop enabled, all of its
dependencies must also be built with C++ interop enabled.

Swift packages that use test discovery get a synthesized
"PackageDiscoveredTests" target which is built by SwiftPM when someone
runs `swift test` command. This module is currently always built without
C++ interop, which is causing issues for projects that rely on it.
### Modifications:

This patch makes sure that the `-cxx-interoperability-mode=` command
line flag gets propagated to the test discovery target.

### Result:

Packages that use both C++ interop and test discovery can now be built
successfully by SwiftPM.

rdar://117078320 / resolves
#6990

Author: egorzhdan

Sources/Build/BuildDescription/SwiftTargetBuildDescription.swift

@@ -555,6 +555,25 @@ public final class SwiftTargetBuildDescription {
             args += ["-color-diagnostics"]
         }
 
+        // If this is a generated test discovery target, it might import a test
+        // target that is built with C++ interop enabled. In that case, the test
+        // discovery target must enable C++ interop as well
+        switch testTargetRole {
+        case .discovery:
+            for dependency in try self.target.recursiveTargetDependencies() {
+                let dependencyScope = self.buildParameters.createScope(for: dependency)
+                let dependencySwiftFlags = dependencyScope.evaluate(.OTHER_SWIFT_FLAGS)
+                if let interopModeFlag = dependencySwiftFlags.first(where: { $0.hasPrefix("-cxx-interoperability-mode=") }) {
+                    args += [interopModeFlag]
+                    break
+                }
+            }
+            if let cxxStandard = self.package.manifest.cxxLanguageStandard {
+                args += ["-Xcc", "-std=\(cxxStandard)"]
+            }
+        default: break
+        }
+
         // Add arguments from declared build settings.
         args += try self.buildSettingsFlags()
 

Tests/BuildTests/BuildPlanTests.swift

@@ -3556,6 +3556,7 @@ final class BuildPlanTests: XCTestCase {
             "/A/Sources/cbar/barcpp.cpp",
             "/A/Sources/cbar/bar.c",
             "/A/Sources/cbar/include/bar.h",
+            "/A/Tests/MySwiftTests/test.swift",
 
             "/B/Sources/t1/dep.swift",
             "/B/Sources/t2/dep.swift",
@@ -3566,6 +3567,7 @@ final class BuildPlanTests: XCTestCase {
             displayName: "A",
             path: "/A",
             toolsVersion: .v5,
+            cxxLanguageStandard: "c++17",
             dependencies: [
                 .localSourceControl(path: "/B", requirement: .upToNextMajor(from: "1.0.0")),
             ],
@@ -3606,6 +3608,12 @@ final class BuildPlanTests: XCTestCase {
                         .init(tool: .linker, kind: .unsafeFlags(["-Ilfoo", "-L", "lbar"])),
                     ]
                 ),
+                try TargetDescription(
+                    name: "MySwiftTests", type: .test,
+                    settings: [
+                        .init(tool: .swift, kind: .interoperabilityMode(.Cxx)),
+                    ]
+                ),
             ]
         )
 
@@ -3682,6 +3690,7 @@ final class BuildPlanTests: XCTestCase {
                     "-Isfoo",
                     "-L", "sbar",
                     "-cxx-interoperability-mode=default",
+                    "-Xcc", "-std=c++17",
                     "-enable-upcoming-feature", "BestFeature",
                     "-g",
                     "-Xcc", "-g",
@@ -3695,6 +3704,9 @@ final class BuildPlanTests: XCTestCase {
 
             let linkExe = try result.buildProduct(for: "exe").linkArguments()
             XCTAssertMatch(linkExe, [.anySequence, "-lsqlite3", "-llibz", "-Ilfoo", "-L", "lbar", "-g", .end])
+
+            let testDiscovery = try result.target(for: "APackageDiscoveredTests").swiftTarget().compileArguments()
+            XCTAssertMatch(testDiscovery, [.anySequence, "-cxx-interoperability-mode=default", "-Xcc", "-std=c++17"])
         }
 
         // omit frame pointers explicitly set to true
@@ -3739,6 +3751,7 @@ final class BuildPlanTests: XCTestCase {
                     "-Isfoo",
                     "-L", "sbar",
                     "-cxx-interoperability-mode=default",
+                    "-Xcc", "-std=c++17",
                     "-enable-upcoming-feature",
                     "BestFeature",
                     "-g",
@@ -3794,6 +3807,7 @@ final class BuildPlanTests: XCTestCase {
                     "-Isfoo",
                     "-L", "sbar",
                     "-cxx-interoperability-mode=default",
+                    "-Xcc", "-std=c++17",
                     "-enable-upcoming-feature",
                     "BestFeature",
                     "-g",
@@ -3836,6 +3850,7 @@ final class BuildPlanTests: XCTestCase {
                     "-Isfoo",
                     "-L", "sbar",
                     "-cxx-interoperability-mode=default",
+                    "-Xcc", "-std=c++17",
                     "-enable-upcoming-feature", "BestFeature",
                     "-enable-upcoming-feature", "WorstFeature",
                     "-g",
@@ -3845,7 +3860,7 @@ final class BuildPlanTests: XCTestCase {
             )
 
             let exe = try result.target(for: "exe").swiftTarget().compileArguments()
-            XCTAssertMatch(exe, [.anySequence, "-DFOO", "-cxx-interoperability-mode=default", "-g", "-Xcc", "-g", .end])
+            XCTAssertMatch(exe, [.anySequence, "-DFOO", "-cxx-interoperability-mode=default", "-Xcc", "-std=c++17", "-g", "-Xcc", "-g", .end])
 
             let linkExe = try result.buildProduct(for: "exe").linkArguments()
             XCTAssertMatch(