More header elimination. The goal of all this is to allow Parser to

More header elimination.  The goal of all this is to allow Parser to
#include Sema.h while keeping all the AST declarations opaque.  That may
not be reasonably attainable, though.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@111907 91177308-0d34-0410-b5e6-96231b3b80d8