Plumb async-ness through StdlibUnittest

This patch plumbs through the ability to run async code in the standard
library unittests. The async start-point is `runAllTestsAsync` and must
be called from an async context. For now, that means calling through
async-main.

Author: etcwilde

stdlib/private/StdlibUnittest/CMakeLists.txt

@@ -45,6 +45,7 @@ add_swift_target_library(swiftStdlibUnittest ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES}
   SWIFT_MODULE_DEPENDS_WINDOWS CRT WinSDK
   SWIFT_COMPILE_FLAGS ${swift_stdlib_unittest_compile_flags} ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
   INSTALL_IN_COMPONENT stdlib-experimental
-  DARWIN_INSTALL_NAME_DIR "${SWIFT_DARWIN_STDLIB_PRIVATE_INSTALL_NAME_DIR}")
+  DARWIN_INSTALL_NAME_DIR "${SWIFT_DARWIN_STDLIB_PRIVATE_INSTALL_NAME_DIR}"
+  LINK_LIBRARIES swift_Concurrency)
 set_source_files_properties(InspectValue.cpp PROPERTIES COMPILE_FLAGS -std=c++14)