[clangd] Get rid of ASTWorker::getCurrentFileInputs

Summary:
FileInputs are only written by ASTWorker thread, therefore it is safe
to read them without the lock inside that thread. It can still be read by other
threads through ASTWorker::getCurrentCompileCommand though.

This patch also gets rid of the smart pointer wrapping FileInputs as there is
never mutliple owners.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77309