[clangd] Symbol index interfaces and an in-memory index implementation.

Summary:
o Index interfaces to support using different index sources (e.g. AST index, global index) for code completion, cross-reference finding etc. This patch focuses on code completion.

The following changes in the original patch has been split out.
o Implement an AST-based index.
o Add an option to replace sema code completion for qualified-id with index-based completion.
o Implement an initial naive code completion index which matches symbols that have the query string as substring.

Reviewers: malaperle, sammccall

Reviewed By: sammccall

Subscribers: hokein, klimek, malaperle, mgorny, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D40548

llvm-svn: 320688