[clangd] (Attempt to) read clang-format file for document formatting

Summary:
Takes into account the clang-format file of the project, if any.
Reverts to LLVM if nothing is found. Replies with an error if any error occured.
For instance, a parse error in the clang-format YAML file.

Reviewers: ilya-biryukov, sammccall, Nebiroth, malaperle, krasimir

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41031

llvm-svn: 320524