Add a -rewrite-includes option, which is similar to -rewrite-macros, but only...

Add a -rewrite-includes option, which is similar to -rewrite-macros, but only expands #include directives.

Patch contributed by Lubos Lunak (l.lunax@suse.cz).
Review by Matt Beaumont-Gay (matthewbg@google.com).

llvm-svn: 158093