[clang-move] Remove clang:: qualifier

Summary:
The use sites are enclosed by `namespace clang`, so clang:: is not
necessary. Many unqualified names have already been used, e.g. SourceManager SourceLocation LangOptions. This change makes the code terser and more consistent.

Reviewers: hokein

Reviewed By: hokein

Subscribers: ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D53060

llvm-svn: 344256