Move PragmaCommentHandler to lib/Parse in preparation for calling Sema

Summary:
No functionality change.  The existing tests for this pragma only verify
that we can preprocess it.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D751

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181246 91177308-0d34-0410-b5e6-96231b3b80d8