Currently -fms-extensions controls this behavior, which doesn't make
much sense. It means we can't identify what is and isn't a system header
when compiling our own preprocessed output, because #line doesn't
represent this information.

If someone is feeding Clang's preprocessed output to another compiler,
they can use this flag.

Fixes PR20553.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D5217

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@230587 91177308-0d34-0410-b5e6-96231b3b80d8