Revert "Move -fms-extensions predefined macros into InitPreprocessor"

This reverts commit r206413.

This was proposed before, but it's not clear if this is really a good
idea:
http://reviews.llvm.org/D3034

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206415 91177308-0d34-0410-b5e6-96231b3b80d8