[clangd] Always retrieve ProjectInfo from Base in OverlayCDB

Summary:
Clangd is returning current working directory for overriden commands.
This can cause inconsistencies between:
- header and the main files, as OverlayCDB only contains entries for the main
  files it direct any queries for the headers to the base, creating a
  discrepancy between the two.
- different clangd instances, as the results will be different depending on the
  timing of execution of the query and override of the command. hence clangd
  might see two different project infos for the same file between different
  invocations.
- editors and the way user has invoked it, as current working directory of
  clangd will depend on those, hence even when there's no underlying base CWD
  might change depending on the editor, or the directory user has started the
  editor in.

This patch gets rid of that discrepency by always directing queries to base or
returning llvm::None in absence of it.

For a sample bug see https://reviews.llvm.org/D83099#2154185.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83934