[clangd] Add an interface that finds symbol by SymbolID in SymbolIndex.

Summary:
Potential use case: argument go-to-definition result with symbol
information (e.g. function definition in cc file) that might not be in the AST.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D44305

llvm-svn: 327487