[clangd] LSP extension to switch between source/header file

Summary:
Small extension to LSP to allow clients to use clangd to switch between C header files and source files.
Final version will use the completed clangd indexer to use the index of symbols to be able to switch from header to source file when the file names don't match.

Reviewers: malaperle, krasimir, bkramer, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: ilya-biryukov, cfe-commits, arphaman

Patch by: William Enright

Differential Revision: https://reviews.llvm.org/D36150

llvm-svn: 314377