[clangd] (Attempt to) read clang-format file for document formatting
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
Differential Revision: https://reviews.llvm.org/D41031