Allow operator keywords to be #defined in ms-ext mode.

Fixes PR10606.

I'm not sure if this is the best way to go about it, but
I locally enabled this code path without the msext conditional,
and all tests pass, except for test/Preprocessor/cxx_oper_keyword.cpp
which explicitly checks that operator keywords can't be redefined.

I also parsed chromium/win with a clang with and without this patch.
It introduced no new errors, but removes 43 existing errors.

llvm-svn: 151768