[clang] Make `FileEntryRef::getDir()` return the as-requested `DirectoryEntryRef`

For redirected file entries, `FileEntryRef::getDir()` returns the parent directory entry of the target file entry. This differs from `FileEntry::getDir()` that always returns the parent directory that was last used to look up that file.

After switching from `FileEntry` to `FileEntryRef` for umbrella headers in D142113, this discrepancy became observable and caused Clang to emit incorrect diagnostics.

This patch changes Clang so that it always associates `FileEntryRef` with the parent directory that was used to look it up. This brings its behavior closer to `FileEntry`, but without the hacky mutation.

This also ensures that `llvm::sys::path::parent_path(FileRef->getNameAsRequested()) == FileRef->getDir()->getName()`. Previously, `FileRef->getDir()` would fall underneath the redirecting VFS into the world of on-disk paths.

Reviewed By: benlangmuir, rmaz

Differential Revision: https://reviews.llvm.org/D151398