Remove Support/IncludeFile.h and its only user. This is actively harmful, since

it breaks the modules builds (where CallGraph.h can be quite reasonably
transitively included by an unimported portion of a module, and CallGraph.cpp
not linked in), and appears to have been entirely redundant since PR780 was
fixed back in 2008.

If this breaks anything, please revert; I have only tested this with a single
configuration, and it's possible that this is still somehow fixing something
(though I doubt it, since no other similar file uses this mechanism any more).

llvm-svn: 215142