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/Support/CrashRecoveryTest.cpp b/llvm/unittests/Support/CrashRecoveryTest.cpp
index 3f13693..ac531b2 100644
--- a/llvm/unittests/Support/CrashRecoveryTest.cpp
+++ b/llvm/unittests/Support/CrashRecoveryTest.cpp
@@ -11,7 +11,7 @@
 #include "llvm/Support/CrashRecoveryContext.h"
 #include "gtest/gtest.h"
 
-#ifdef LLVM_ON_WIN32
+#ifdef _WIN32
 #define WIN32_LEAN_AND_MEAN
 #define NOGDI
 #include <windows.h>
@@ -61,7 +61,7 @@
   EXPECT_EQ(1, GlobalInt);
 }
 
-#ifdef LLVM_ON_WIN32
+#ifdef _WIN32
 static void raiseIt() {
   RaiseException(123, EXCEPTION_NONCONTINUABLE, 0, NULL);
 }