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/CrashRecoveryContext.cpp b/llvm/lib/Support/CrashRecoveryContext.cpp
index bd38dd8..f389440 100644
--- a/llvm/lib/Support/CrashRecoveryContext.cpp
+++ b/llvm/lib/Support/CrashRecoveryContext.cpp
@@ -189,7 +189,7 @@
 
 #else // !_MSC_VER
 
-#if defined(LLVM_ON_WIN32)
+#if defined(_WIN32)
 // This is a non-MSVC compiler, probably mingw gcc or clang without
 // -fms-extensions. Use vectored exception handling (VEH).
 //
@@ -272,7 +272,7 @@
   }
 }
 
-#else // !LLVM_ON_WIN32
+#else // !_WIN32
 
 // Generic POSIX implementation.
 //
@@ -342,7 +342,7 @@
     sigaction(Signals[i], &PrevActions[i], nullptr);
 }
 
-#endif // !LLVM_ON_WIN32
+#endif // !_WIN32
 
 bool CrashRecoveryContext::RunSafely(function_ref<void()> Fn) {
   // If crash recovery is disabled, do nothing.