The config providers that look for configuration files currently take a pointer to a FileSystem in the constructor.
For some reason this isn't actually used when trying to read those configuration files, Essentially it just follows the behaviour of the real filesystem.
Using clang-tidy standalone this doesn't cause any issue.
But if its used as a library and the user wishes to use say an `InMemoryFileSystem` it will try to read the files from the disc instead.
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D90992
Kazu Hirata