This broke e.g. compiling a crash report from a glibc system on Darwin. Sadly,
the implementation had to game the lexer a lot as we're not using a real
preprocessor here. It also doesn't handle special cases like arbitrary macros in
__has_include, but since this macro isn't common outside of clang's headers we
can get away with that.

Fixes PR14422.

Differential Revision: http://llvm-reviews.chandlerc.com/D594

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179616 91177308-0d34-0410-b5e6-96231b3b80d8