[clang-format] Remove the dependency on frontend
Address review comments from D68554: [clang-format] Proposal for clang-format to give compiler style warnings 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