[clang][DependencyScanner] Cache modulemap stat failures

Add `module.modulemap` as a file we cache stat failures for as there
are a lot of stats for this file.

Clang currently uses the files it should minimize as a proxy for files
it should cache stat failures for, but really we should cache stat
failures for all paths we don't expect to change during the build.
Unfortunately the VFS API does not know _why_ clang is trying to stat
a path, so we use the filename as a proxy.

Differential Revision: https://reviews.llvm.org/D145526