Add callout for FoundationEssentials to handle non-UTF encodings (swiftlang#5193)

Author: parkera

Sources/Foundation/NSString.swift

@@ -7,7 +7,7 @@
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 
-
+@_spi(SwiftCorelibsFoundation) @_exported import FoundationEssentials
 @_implementationOnly import CoreFoundation
 internal import Synchronization
 
@@ -1669,3 +1669,14 @@ extension String : CVarArg, _CVarArgObject {
     }
 }
 #endif
+
+// Upcall from swift-foundation for conversion of less frequently-used encodings
+@_dynamicReplacement(for: _cfStringEncodingConvert(string:using:allowLossyConversion:))
+private func _cfStringEncodingConvert_corelibs_foundation(string: String, using encoding: UInt, allowLossyConversion: Bool) -> Data? {
+    return (string as NSString).data(using: encoding, allowLossyConversion: allowLossyConversion)
+}
+
+@_dynamicReplacement(for: _cfMakeStringFromBytes(_:encoding:))
+private func _cfMakeStringFromBytes_corelibs_foundation(_ bytes: UnsafeBufferPointer<UInt8>, encoding: UInt) -> String? {
+    return NSString(bytes: bytes.baseAddress!, length: bytes.count, encoding: encoding) as? String
+}

Tests/Foundation/TestNSString.swift

@@ -1725,4 +1725,16 @@ class TestNSString: LoopbackServerTest {
         XCTAssertNotNil(str)
         XCTAssertEqual(str?.isEmpty, true)
     }
+    
+    func test_windows1252Encoding() {
+        // Define an array of CP1252 encoded bytes representing "Hallo " followed by the Euro sign
+        let cp1252Bytes: [UInt8] = [72, 97, 108, 108, 111, 32, 0x80]
+        let cp1252Data = Data(cp1252Bytes)
+
+        let nativeString = String(data: cp1252Data, encoding: .windowsCP1252)
+        XCTAssertEqual(nativeString, "Hallo €")
+        
+        let producedData = nativeString?.data(using: .windowsCP1252)
+        XCTAssertEqual(producedData, cp1252Data)
+    }
 }