SwiftRemoteMirror: do not mark as weak import on Windows

Weak import semantics are not available on PE/COFF.  Ensure that we do not mark
the type as having weak import semantics.  Otherwise, the dllimport'ed symbol is
marked as `dso_local` which is invalid.

Author: compnerd

include/swift/SwiftRemoteMirror/SwiftRemoteMirror.h

@@ -37,7 +37,10 @@
 extern "C" {
 #endif
 
-SWIFT_REMOTE_MIRROR_LINKAGE __attribute__((__weak_import__))
+SWIFT_REMOTE_MIRROR_LINKAGE
+#if !defined(_WIN32)
+__attribute__((__weak_import__))
+#endif
 extern unsigned long long swift_reflection_classIsSwiftMask;
 
 /// Get the metadata version supported by the Remote Mirror library.