[SourceMgr] Tidy up the SourceMgr header file to include less stuff.

Summary:
Specifically make some simple refactorings to get PointerUnion.h and
Twine.h out of the public includes.  While here, trim out a lot of
transitive includes as well.

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78870