Enable WindowsToolChain on all native Windows builds

Make binaries built by MSVC, mingw and clang functionally equivalent. The
checks are trivially performed at runtime to eliminate functional differences
between supported configurations that used to be hard-coded.

llvm-svn: 211461

clang/lib/Driver/WindowsToolChain.cpp

@@ -14,6 +14,7 @@
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
+#include "llvm/Config/llvm-config.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -21,8 +22,7 @@
 
 // Include the necessary headers to interface with the Windows registry and
 // environment.
-// TODO: Investigate enabling this with __MINGW32__.
-#if defined(_MSC_VER)
+#if defined(LLVM_ON_WIN32)
 #define USE_WIN32
 #endif
 
@@ -283,11 +283,6 @@ void Windows::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
   if (DriverArgs.hasArg(options::OPT_nostdlibinc))
     return;
 
-// TODO: This code path is portable. Conditionalize on LLVM_ON_WIN32 instead?
-#ifndef USE_WIN32
-  return;
-#endif
-
   // Honor %INCLUDE%. It should know essential search paths with vcvarsall.bat.
   if (const char *cl_include_dir = getenv("INCLUDE")) {
     SmallVector<StringRef, 8> Dirs;