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

[Alexandre Ganea via cfe-commits]

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@@ -201,6 +201,21 @@ class ErrorDummyFileSystem : public DummyFileSystem {
   }
 };
 
+/// A version of \c DummyFileSystem that aborts on \c status() to test that
+/// \c exists() is being used.
+class NoStatusDummyFileSystem : public DummyFileSystem {
+public:
+  ErrorOr<vfs::Status> status(const Twine &Path) override {
+    llvm::report_fatal_error(
+        "unexpected call to NoStatusDummyFileSystem::status");
+  }
+
+  bool exists(const Twine &Path) override {
+    auto Status = DummyFileSystem::status(Path);
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aganea wrote:

Explicit return type here too.

https://github.com/llvm/llvm-project/pull/88152