s/LLVM_ON_WIN32/_WIN32/, clang

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.

llvm-svn: 331069
diff --git a/clang/lib/Frontend/PrecompiledPreamble.cpp b/clang/lib/Frontend/PrecompiledPreamble.cpp
index ddd7bda..23371d1 100644
--- a/clang/lib/Frontend/PrecompiledPreamble.cpp
+++ b/clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -40,7 +40,7 @@
 StringRef getInMemoryPreamblePath() {
 #if defined(LLVM_ON_UNIX)
   return "/__clang_tmp/___clang_inmemory_preamble___";
-#elif defined(LLVM_ON_WIN32)
+#elif defined(_WIN32)
   return "C:\\__clang_tmp\\___clang_inmemory_preamble___";
 #else
 #warning "Unknown platform. Defaulting to UNIX-style paths for in-memory PCHs"