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
llvm-svn: 230587
Reid Kleckner