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
Volodymyr Sapsai