Add a callback for `__has_include` and use it for dependency scanning.

This adds a preprocessor callback for the `__has_include` and
`__has_include_next` directives.

Successful checking for the presence of a header should add it to the list of
header dependencies so this overrides the callback in the dependency scanner.

Patch by Pete Cooper with some additions by me.

rdar://problem/39545636

Differential Revision: https://reviews.llvm.org/D30882

llvm-svn: 342517