rewrite-includes: Rewrite __has_include(_next) to get rid of a host dependency.

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