MS ABI: Refactor extended qualifiers

Extended qualifiers can appear in many places, refactor the code so it's
more reusable.  Add tests in areas where we've increased compatibility.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@201574 91177308-0d34-0410-b5e6-96231b3b80d8