[clangd] Suppress GCC -Woverloaded-virtual by renaming ThreadsafeFS extension point

Summary:
By making all overloads non-virtual and delegating to a differently-named
private method, we avoid any (harmless) name-hiding in the subclasses.

Reviewers: kadircet

Subscribers: kristof.beyls, ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits, Quuxplusone, dblaikie

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82793