[clang/HeaderSearch] Make sure `loadSubdirectoryModuleMaps` doesn't cause loading of regular files

`HeaderSearch::loadSubdirectoryModuleMaps` `stat`s all the files in a directory which causes the dependency scanning
service to load and cache their contents. This is problematic because a file may be in the process of being generated
and could be cached by the dep-scan service while it is still incomplete.

To address this change `loadSubdirectoryModuleMaps` to ignore regular files.

Differential Revision: https://reviews.llvm.org/D153670