[clangd] Rename FSProvider to TFS in case of ThreadsafeFS

Summary: Depends on D81998

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82024