[clang-format] Improve Incomplete detection for (text) protos

Summary:
This patch improves detection of incomplete code for protos and text protos.
This is especially important for text protos in raw string literals, since they
might be partial strings concatenated, and we'd like to disable formatting in
these cases.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D44203

llvm-svn: 330016