Add Clang docs about MSVC compatibility

This documents some of the status of supported functionality in MSVC
quirks mode.  Some of this should be in
http://clang.llvm.org/compatibility.html instead when things have
stabilized.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202559 91177308-0d34-0410-b5e6-96231b3b80d8