Pre-declare '::type_info' in MicrosoftMode only, not MicrosoftExt

It was previously enabled in both but should only have been part of the drop-in
quirks mode that is 'MicrosoftMode' given that it's only useful for
compatibility with the Microsoft headers/runtime.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198548 91177308-0d34-0410-b5e6-96231b3b80d8