Use weak_import not weak for the optional symbols

Author: aschwaighofer

include/swift/Runtime/Exclusivity.h

@@ -45,7 +45,7 @@ void swift_beginAccess(void *pointer, ValueBuffer *buffer,
 /// Returns null if the original function (which is passed in \p CurrFn) should
 /// be called.
 #ifdef __APPLE__
-__attribute__((weak))
+__attribute__((weak_import))
 #endif
 SWIFT_RUNTIME_EXPORT
 char *swift_getFunctionReplacement(char **ReplFnPtr, char *CurrFn);
@@ -55,7 +55,7 @@ char *swift_getFunctionReplacement(char **ReplFnPtr, char *CurrFn);
 /// This function is called from a replacement function to call the original
 /// function.
 #ifdef __APPLE__
-__attribute__((weak))
+__attribute__((weak_import))
 #endif
 SWIFT_RUNTIME_EXPORT
 char *swift_getOrigOfReplaceable(char **OrigFnPtr);