[test] Add new libdispatch -vfsoverlay flag for linux, as in swiftlang/swift-corelibs-libdispatch#785

This allows the tests that use libdispatch to find its modulemap, plus add the
libdispatch compilation flags to one test that was missing them and fix one
async test on linux.

Author: finagolfin

test/Concurrency/Runtime/async_task_priority_current.swift

@@ -10,17 +10,10 @@
 
 import Dispatch
 
-// Work around the inability of older Swift runtimes to print a task priority.
-extension TaskPriority: CustomStringConvertible {
-  public var description: String {
-    "TaskPriority(rawValue: \(rawValue))"
-  }
-}
-
 @available(SwiftStdlib 5.1, *)
 @main struct Main {
   static func main() async {
-    print("main priority: \(Task.currentPriority)") // CHECK: main priority: TaskPriority(rawValue: [[#MAIN_PRIORITY:]])
+    print("main priority: \(Task.currentPriority.rawValue)") // CHECK: main priority: [[#MAIN_PRIORITY:]]
     await test_detach()
     await test_multiple_lo_indirectly_escalated()
   }
@@ -29,18 +22,18 @@ extension TaskPriority: CustomStringConvertible {
 @available(SwiftStdlib 5.1, *)
 func test_detach() async {
   let a1 = Task.currentPriority
-  print("a1: \(a1)") // CHECK: a1: TaskPriority(rawValue: [[#MAIN_PRIORITY]])
+  print("a1: \(a1.rawValue)") // CHECK: a1: [[#MAIN_PRIORITY]]
 
   // Note: remember to detach using a higher priority, otherwise a lower one
   // might be escalated by the get() and we could see `default` in the detached
   // task.
   await detach(priority: .userInitiated) {
     let a2 = Task.currentPriority
-    print("a2: \(a2)") // CHECK: a2: TaskPriority(rawValue: 25)
+    print("a2: \(a2.rawValue)") // CHECK: a2: 25
   }.get()
 
   let a3 = Task.currentPriority
-  print("a3: \(a3)") // CHECK: a3: TaskPriority(rawValue: [[#MAIN_PRIORITY]])
+  print("a3: \(a3.rawValue)") // CHECK: a3: [[#MAIN_PRIORITY]]
 }
 
 @available(SwiftStdlib 5.1, *)

test/Concurrency/Runtime/custom_executors_complex_equality_crash.swift

@@ -1,4 +1,4 @@
-// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library)
+// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking %import-libdispatch -parse-as-library)
 
 // REQUIRES: concurrency
 // REQUIRES: executable_test

test/lit.cfg

@@ -1643,15 +1643,17 @@ elif (run_os in ['linux-gnu', 'linux-gnueabihf', 'freebsd', 'openbsd', 'windows-
     libdispatch_artifact_dir = config.libdispatch_build_path
     libdispatch_swift_module_dir = make_path(libdispatch_artifact_dir, 'src', 'swift', 'swift')
     libdispatch_source_dir = make_path(config.swift_src_root, os.pardir, 'swift-corelibs-libdispatch')
+    libdispatch_vfs_yaml =  make_path(libdispatch_artifact_dir, 'dispatch-vfs-overlay.yaml')
     libdispatch_artifacts = [
+        libdispatch_vfs_yaml,
         make_path(libdispatch_artifact_dir, 'libdispatch.so'),
         make_path(libdispatch_artifact_dir, 'libswiftDispatch.so'),
         make_path(libdispatch_swift_module_dir, 'Dispatch.swiftmodule')]
     if (all(os.path.exists(p) for p in libdispatch_artifacts)):
         config.available_features.add('libdispatch')
         config.libdispatch_artifact_dir = libdispatch_artifact_dir
-        config.import_libdispatch = ('-I %s -I %s -L %s'
-            % (libdispatch_source_dir, libdispatch_swift_module_dir, libdispatch_artifact_dir))
+        config.import_libdispatch = ('-I %s -I %s -L %s -vfsoverlay %s'
+            % (libdispatch_source_dir, libdispatch_swift_module_dir, libdispatch_artifact_dir, libdispatch_vfs_yaml))
 
     libdispatch_static_artifact_dir = os.path.join(config.libdispatch_static_build_path, 'lib')
     libdispatch_static_artifacts = [