[Long runtime tests] Link the Objective-C runtime.

The long-running runtime tests depend on the Objective-C runtime on
Darwin, so link it directly rather than relying on various uses of
__builtin_available to link CoreFoundation for us.

Author: DougGregor

unittests/runtime/LongTests/CMakeLists.txt

@@ -16,6 +16,9 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
 #      ${FOUNDATION_LIBRARY}
 #      swiftStdlibUnittest${SWIFT_PRIMARY_VARIANT_SUFFIX}
 #      )
+
+    # Link the Objective-C runtime.
+    list(APPEND PLATFORM_TARGET_LINK_LIBRARIES "objc")
   elseif(SWIFT_HOST_VARIANT STREQUAL "linux")
     list(APPEND PLATFORM_TARGET_LINK_LIBRARIES
       "atomic"