Downgrade ext_enumerator_too_large from ExtWarn to Extension in Microsoft...

Downgrade ext_enumerator_too_large from ExtWarn to Extension in Microsoft mode. Otherwise you can warnings flooding trying to selfhost clang with fms-extensions because of "unsigned int" -> "signed int" enumerator conversion.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123600 91177308-0d34-0410-b5e6-96231b3b80d8