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/tools/driver/driver.cpp b/clang/tools/driver/driver.cpp
index 1913996..c83e103 100644
--- a/clang/tools/driver/driver.cpp
+++ b/clang/tools/driver/driver.cpp
@@ -485,7 +485,7 @@
       // On Windows, abort will return an exit code of 3.  In these cases,
       // generate additional diagnostic information if possible.
       bool DiagnoseCrash = CommandRes < 0 || CommandRes == 70;
-#ifdef LLVM_ON_WIN32
+#ifdef _WIN32
       DiagnoseCrash |= CommandRes == 3;
 #endif
       if (DiagnoseCrash) {
@@ -501,7 +501,7 @@
   // results now.  This happens in -disable-free mode.
   llvm::TimerGroup::printAll(llvm::errs());
 
-#ifdef LLVM_ON_WIN32
+#ifdef _WIN32
   // Exit status should not be negative on Win32, unless abnormal termination.
   // Once abnormal termiation was caught, negative status should not be
   // propagated.