Add -fseh-exceptions for MinGW-w64

This adds a flag called -fseh-exceptions that uses the native Windows
.pdata and .xdata unwind mechanism to throw exceptions. The other EH
possibilities are DWARF and SJLJ exceptions.

Patch by Martell Malone!

Reviewed By: asl, rnk

Differential Revision: http://reviews.llvm.org/D3419

llvm-svn: 217790

clang/lib/Frontend/InitPreprocessor.cpp

@@ -556,6 +556,8 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
     Builder.defineMacro("__GXX_RTTI");
   if (LangOpts.SjLjExceptions)
     Builder.defineMacro("__USING_SJLJ_EXCEPTIONS__");
+  if (LangOpts.SEHExceptions)
+    Builder.defineMacro("__SEH__");
 
   if (LangOpts.Deprecated)
     Builder.defineMacro("__DEPRECATED");