[Headers] Expand _Unwind_Exception for SEH on MinGW/x86_64

This matches how GCC defines this struct.

Differential Revision: https://reviews.llvm.org/D50380

llvm-svn: 339170
diff --git a/clang/lib/Headers/unwind.h b/clang/lib/Headers/unwind.h
index 345fa4d..0e8317e 100644
--- a/clang/lib/Headers/unwind.h
+++ b/clang/lib/Headers/unwind.h
@@ -154,8 +154,12 @@
 struct _Unwind_Exception {
   _Unwind_Exception_Class exception_class;
   _Unwind_Exception_Cleanup_Fn exception_cleanup;
+#if !defined (__USING_SJLJ_EXCEPTIONS__) && defined (__SEH__)
+  _Unwind_Word private_[6];
+#else
   _Unwind_Word private_1;
   _Unwind_Word private_2;
+#endif
   /* The Itanium ABI requires that _Unwind_Exception objects are "double-word
    * aligned".  GCC has interpreted this to mean "use the maximum useful
    * alignment for the target"; so do we. */