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

[Jan Svoboda via cfe-commits]

================
@@ -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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jansvoboda11 wrote:

I guess this could be inlined into something like
```c++
llvm::StringMap<std::pair<const CachedFileSystemEntry *, const CachedRealPath *>> InfoByFilename
```

Any preferences?

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