Don't #include ClangASTContext.h from Module.h

This is part of a larger effort to reduce header file footprints.
Combined, these patches reduce the build time of LLDB locally by
over 30%.  However, they touch many files and make many changes,
so will be submitted in small incremental pieces.

Reviewed By: Greg Clayton
Differential Revision: http://reviews.llvm.org/D8022

llvm-svn: 231097