StdlibUnittest: import _Concurrency explicitly (#39175)

StdlibUnittest is built with `-disable-implicit-concurrency-module-import`
and it uses concurrency feature. Therefore, without explicit import,
StdlibUnittest.swiftmodule doesn't have `IMPORTED_MODULE` entry of
`_Concurrency`. And it results link-failure when static linking because
swift_Concurrency is not linked.

Co-authored-by: Yuta Saito <[email protected]>

Author: MaxDesiatov

stdlib/private/StdlibUnittest/StdlibUnittest.swift

@@ -31,6 +31,10 @@ import WinSDK
 import ObjectiveC
 #endif
 
+#if SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY
+import _Concurrency
+#endif
+
 extension String {
   /// Returns the lines in `self`.
   public var _lines : [String] {