Move -fms-extensions predefined macros into InitPreprocessor

If someone on Linux asks for -fms-extensions, there's no reason not to
define the feature test macros that MSVC defines.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206413 91177308-0d34-0410-b5e6-96231b3b80d8