Summary: This patch adds support for textual protocol buffer messages.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek, mgorny

Differential Revision: https://reviews.llvm.org/D34441

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307029 91177308-0d34-0410-b5e6-96231b3b80d8