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/InitHeaderSearch.cpp b/clang/lib/Frontend/InitHeaderSearch.cpp
index 0506975..d2fcc9a 100644
--- a/clang/lib/Frontend/InitHeaderSearch.cpp
+++ b/clang/lib/Frontend/InitHeaderSearch.cpp
@@ -105,7 +105,7 @@
 }  // end anonymous namespace.
 
 static bool CanPrefixSysroot(StringRef Path) {
-#if defined(LLVM_ON_WIN32)
+#if defined(_WIN32)
   return !Path.empty() && llvm::sys::path::is_separator(Path[0]);
 #else
   return llvm::sys::path::is_absolute(Path);