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