[clangd] Add support for per-file extra flags

Summary:
This patch adds the ability to specify user-defined extra flags per opened file
through the LSP layer. This is a non-standard extension to the protocol.
I've already created a feature request about it for upstream lsp:
https://github.com/Microsoft/language-server-protocol/issues/255

The particular use-case is ycmd, which has a python script for figuring out
extra flags per file:
https://github.com/Valloric/ycmd#flagsforfile-filename-kwargs-

Reviewers: ilya-biryukov, klimek, bkramer

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D34947

llvm-svn: 307241