[Support] Make LLVM_EXTERNAL_VISIBILITY meaningful on Windows

[junaire]

Summary

Historically for some reasons LLVM_EXTERNAL_VISIBILITY is a nop on
Windows. This patchs adds support for Windows by marking that as an
alias of __declspec(dllexport).

Signed-off-by: Jun Zhang <[e-mail]>

[junaire]

This should fix Windows CI failures in https://reviews.llvm.org/D141215

[junaire]

Fix the build failure

[junaire]

Better to keep these in clang-repl itself since things get pretty tricky when involved templates...

llvm/include/llvm/Support/Compiler.h

#if __has_attribute(visibility) &&                                             \
      (defined(__MINGW32__) && defined(__clang__)))
 #define LLVM_LIBRARY_VISIBILITY __attribute__ ((visibility("hidden")))
 #if defined(LLVM_BUILD_LLVM_DYLIB) || defined(LLVM_BUILD_SHARED_LIBS)
 #define LLVM_EXTERNAL_VISIBILITY __attribute__((visibility("default")))
 #else
 #define LLVM_EXTERNAL_VISIBILITY
 #endif
 #else
-#define LLVM_LIBRARY_VISIBILITY
+#if defined(_WIN32)
+#define LLVM_EXTERNAL_VISIBILITY __declspec(dllexport)
+#esle
 #define LLVM_EXTERNAL_VISIBILITY
 #endif
+#define LLVM_LIBRARY_VISIBILITY
+#endif
 
 #if defined(__GNUC__)
 #define LLVM_PREFETCH(addr, rw, locality) __builtin_prefetch(addr, rw, locality)
 #else
 #define LLVM_PREFETCH(addr, rw, locality)
 #endif
 
 #if __has_attribute(used)