Make sure we define wchar_t related macros correctly in -fms-extensions mode.

This adds a test to make sure we define _WCHAR_T_DEFINED and
_NATIVE_WCHAR_T_DEFINED correctly in the preprocessor, and updates
stddef.h to set it when typedeffing wchar_t.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@180918 91177308-0d34-0410-b5e6-96231b3b80d8