[clang][deps] Only bypass scanning VFS for the module cache (#88800)
The scanning VFS doesn't cache stat failures of paths with no extension.
This was originally implemented to avoid caching the non-existence of
the modules cache directory that the modular scanner will eventually
create if it does not exist.
However, this prevents caching of the non-existence of all directories
and notably also header files from the standard C++ library, which can
lead to sub-par performance.
This patch adds an API to the scanning VFS that allows clients to
configure path prefix for which to bypass the scanning VFS and use the
underlying VFS directly.
llvm-project
55323ca6 - [clang][deps] Only bypass scanning VFS for the module cache (#88800)
