[clangd] Make it possible to use VFS from parsing for getting tidy options

Summary:
To give an option for clangd embedders with snapshotted filesystem to
read config files from exact snapshots, possibly loosing some
performance from caching capabilities of the current implementations.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62143

llvm-svn: 361178