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

[Ben Langmuir via cfe-commits]

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@@ -230,6 +251,26 @@ class DependencyScanningFilesystemLocalCache {
     assert(InsertedEntry == &Entry && "entry already present");
     return *InsertedEntry;
   }
+
+  /// Returns real path associated with the filename or nullptr if none is
+  /// found.
+  const CachedRealPath *findRealPathByFilename(StringRef Filename) const {
+    assert(llvm::sys::path::is_absolute_gnu(Filename));
+    auto It = RealPathCache.find(Filename);
+    return It == RealPathCache.end() ? nullptr : It->getValue();
+  }
+
+  /// Associates the given real path with the filename and returns the given
+  /// entry pointer (for convenience).
+  const CachedRealPath &
+  insertRealPathForFilename(StringRef Filename,
+                            const CachedRealPath &RealPath) {
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benlangmuir wrote:

Okay, I'm okay with being consistent for now, but I think it's a bad pattern in general to pass around `const X &` and keep a pointer to it.

https://github.com/llvm/llvm-project/pull/68645