Fix clang/test/Sema/struct-packed-align.c. "Windows" is not MS compiler.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254614 91177308-0d34-0410-b5e6-96231b3b80d8

test/Sema/struct-packed-align.c

@@ -151,7 +151,7 @@ struct packed_chars {
   char c:4;
 };
 
-#if defined(_WIN32)
+#if defined(_WIN32) && !defined(__declspec) // _MSC_VER is unavailable in cc1.
 // On Windows clang uses MSVC compatible layout in this case.
 extern int o1[sizeof(struct packed_chars) == 3 ? 1 : -1];
 extern int o2[__alignof(struct packed_chars) == 1 ? 1 : -1];