[clang][deps][test] Update reference to tracing VFS

This fixes build failure introduced by 70fcdb3d52272eba96d4b268d334826690524f50.

[llvm][support] Implement tracing virtual file system (#88326)

LLVM-based tools often use the `llvm::vfs::FileSystem` instrastructure
to access the file system. This patch adds new kind of a VFS th

[llvm][support] Implement tracing virtual file system (#88326)

LLVM-based tools often use the `llvm::vfs::FileSystem` instrastructure
to access the file system. This patch adds new kind of a VFS that
performs lightweight tracing of file system operations on an underlying
VFS. This is supposed to aid in investigating file system traffic
without resorting to instrumentation on the operating system level.
There will be follow-up patches that integrate this into Clang and its
dependency scanner.

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[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 no

[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.

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[clang][deps] Overload `Filesystem::exists` in `DependencyScanningFilesystem` to have it use cached `status` (#88152)

As-is, calls to `exists()` fallback on the implementation in
`ProxyFileSystem::

[clang][deps] Overload `Filesystem::exists` in `DependencyScanningFilesystem` to have it use cached `status` (#88152)

As-is, calls to `exists()` fallback on the implementation in
`ProxyFileSystem::exists` which explicitly calls out to the underlying
`FS`, which for the `DependencyScanningFilesystem` (overlay) is the real
underlying filesystem.

Instead, directly overloading `exists` allows us to have it rely on the
cached `status` behavior used elsewhere by the
`DependencyScanningFilesystem`.

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[clang] Remove unused variable in test

[clang][deps] Cache `VFS::getRealPath()` (#68645)

This PR starts caching calls to
`DependencyScanningWorkerFilesystem::getRealPath()` that we use whenever
we canonicalize module map path. In the c

[clang][deps] Cache `VFS::getRealPath()` (#68645)

This PR starts caching calls to
`DependencyScanningWorkerFilesystem::getRealPath()` that we use whenever
we canonicalize module map path. In the case of the real VFS, this
functions performs an expensive syscall that we'd like to do as rarely
as possible.

This PR keeps the real path out of `CachedFileSystemEntry`, since that's
**immutable**; populating the real path on creation of this data
structure (every stat/open) would be expensive.

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