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/lib/Support/Process.cpp b/llvm/lib/Support/Process.cpp
index 1c8cc6e..d5f9bde 100644
--- a/llvm/lib/Support/Process.cpp
+++ b/llvm/lib/Support/Process.cpp
@@ -93,6 +93,6 @@
 #ifdef LLVM_ON_UNIX
 #include "Unix/Process.inc"
 #endif
-#ifdef LLVM_ON_WIN32
+#ifdef _WIN32
 #include "Windows/Process.inc"
 #endif