Revert "[cmake] Ignore required explicit availability in Swift stdlib"

This reverts commit 4bec9d5.

Author: xymus

stdlib/cmake/modules/SwiftSource.cmake

@@ -483,8 +483,7 @@ function(_compile_swift_files
   # The standard library and overlays are built resiliently when SWIFT_STDLIB_STABLE_ABI=On.
   if(SWIFTFILE_IS_STDLIB AND SWIFT_STDLIB_STABLE_ABI)
     list(APPEND swift_flags "-enable-library-evolution")
-    list(APPEND swift_flags "-library-level" "api")
-    list(APPEND swift_flags "-Xfrontend" "-require-explicit-availability=ignore")
+    list(APPEND swift_flags "-Xfrontend" "-library-level"  "-Xfrontend" "api")
   endif()
 
   if("${SWIFT_SDK_${SWIFTFILE_SDK}_THREADING_PACKAGE}" STREQUAL "none")