[clangd] Use identifiers in file as completion candidates when build is not ready.
o Lex the code to get the identifiers and put them into a "symbol" index.
o Adds a new completion mode without compilation/sema into code completion workflow.
o Make IncludeInserter work even when no compile command is present, by avoiding
inserting non-verbatim headers.
Reviewed By: sammccall
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, jdoerfert, cfe-commits
Differential Revision: https://reviews.llvm.org/D60126