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/unittests/Driver/ToolChainTest.cpp b/clang/unittests/Driver/ToolChainTest.cpp
index 93cf12b..d4198ea 100644
--- a/clang/unittests/Driver/ToolChainTest.cpp
+++ b/clang/unittests/Driver/ToolChainTest.cpp
@@ -69,7 +69,7 @@
     llvm::raw_string_ostream OS(S);
     C->getDefaultToolChain().printVerboseInfo(OS);
   }
-#if LLVM_ON_WIN32
+#if _WIN32
   std::replace(S.begin(), S.end(), '\\', '/');
 #endif
   EXPECT_EQ(
@@ -109,7 +109,7 @@
     llvm::raw_string_ostream OS(S);
     C->getDefaultToolChain().printVerboseInfo(OS);
   }
-#if LLVM_ON_WIN32
+#if _WIN32
   std::replace(S.begin(), S.end(), '\\', '/');
 #endif
   EXPECT_EQ("Found candidate GCC installation: "