Build: search libraries in Apple SDKs (if needed)

Addresses rdar://problem/56204969

Author: edymtt

cmake/modules/AddSwift.cmake

@@ -1261,6 +1261,10 @@ function(_add_swift_library_single target name)
       "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/../lib/swift/${SWIFTLIB_SINGLE_SUBDIR}"
       "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/../lib/swift/${SWIFT_SDK_${SWIFTLIB_SINGLE_SDK}_LIB_SUBDIR}")
 
+  if(DEFINED ENV{SDKROOT})
+      list(APPEND library_search_directories "$ENV{SDKROOT}/usr/lib/swift")
+  endif()
+
   # Add variant-specific flags.
   if(SWIFTLIB_SINGLE_TARGET_LIBRARY)
     set(build_type "${SWIFT_STDLIB_BUILD_TYPE}")