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

[Ben Langmuir via cfe-commits]

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@@ -168,6 +170,12 @@ class DependencyScanningFilesystemSharedCache {
     /// The backing storage for cached contents.
     llvm::SpecificBumpPtrAllocator<CachedFileContents> ContentsStorage;
 
+    /// Map from filenames to cached real paths.
+    llvm::StringMap<const CachedRealPath *> RealPathsByFilename;
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benlangmuir wrote:

I would lean towards combining them unless there's a drawback I'm not seeing - the keys are fairly large here so it's nice to share storage for that.  I would maybe feel differently for a DenseMap of small keys :shrug:

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