[clang][deps] Overload 'Filesystem::exists' in 'DependencyScanningFilesystem' to have it use cached status

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'.

Author: artemcm

clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h

@@ -365,6 +365,10 @@ class DependencyScanningWorkerFilesystem
     return LocalCache.insertEntryForFilename(Filename, Entry);
   }
 
+  /// Check whether \p Path exists. By default checks cached result of \c status(),
+  /// and falls back on FS if unable to do so.
+  bool exists(const Twine &Path) override;
+
   /// Returns entry associated with the filename in the shared cache if there is
   /// some. Otherwise, constructs new one with the given error code, associates
   /// it with the filename and returns the result.

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

@@ -270,6 +270,12 @@ DependencyScanningWorkerFilesystem::status(const Twine &Path) {
   return Result->getStatus();
 }
 
+bool
+DependencyScanningWorkerFilesystem::exists(const Twine &Path) {
+  auto Status = status(Path);
+  return Status && Status->exists();
+}
+
 namespace {
 
 /// The VFS that is used by clang consumes the \c CachedFileSystemEntry using