s/LLVM_ON_WIN32/_WIN32/, llvm

LLVM_ON_WIN32 is set exactly with MSVC and MinGW (but not Cygwin) in
HandleLLVMOptions.cmake, which is where _WIN32 defined too.  Just use the
default macro instead of a reinvented one.

See thread "Replacing LLVM_ON_WIN32 with just _WIN32" on llvm-dev and cfe-dev.
No intended behavior change.

This moves over all uses of the macro, but doesn't remove the definition
of it in (llvm-)config.h yet.

llvm-svn: 331127
diff --git a/llvm/unittests/IR/ValueMapTest.cpp b/llvm/unittests/IR/ValueMapTest.cpp
index 28633b4..cbcd6f6 100644
--- a/llvm/unittests/IR/ValueMapTest.cpp
+++ b/llvm/unittests/IR/ValueMapTest.cpp
@@ -195,7 +195,7 @@
   static MutexT *getMutex(const ExtraData &Data) { return Data.M; }
 };
 // FIXME: These tests started failing on Windows.
-#if LLVM_ENABLE_THREADS && !defined(LLVM_ON_WIN32)
+#if LLVM_ENABLE_THREADS && !defined(_WIN32)
 TYPED_TEST(ValueMapTest, LocksMutex) {
   sys::Mutex M(false);  // Not recursive.
   bool CalledRAUW = false, CalledDeleted = false;