[clangd] Add a textual fallback for go-to-definition

Summary:
This facilitates performing go-to-definition in contexts where AST-based
resolution does not work, such as comments, string literals, preprocessor
disabled regions, and macro definitions, based on textual lookup in the index.

Partially fixes https://github.com/clangd/clangd/issues/241

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72874