[clang-format] Remove the dependency on frontend

Summary:
Address review comments from {D68554} by trying to drop the dependency again on Frontend whilst keeping the same format diagnostic messages

Not completely happy with having to do a split in order to get the StringRef for the Line the error occurred on, but could see a way to use SourceManager and SourceLocation to give me a single line?

But this removes the dependency on frontend which should keep the binary size down.

Reviewers: thakis, klimek, mitchell-stellar

Reviewed By: klimek

Subscribers: mgorny, cfe-commits

Tags: #clang, #clang-format

Differential Revision: https://reviews.llvm.org/D68969