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

[Jan Svoboda via cfe-commits]

================
@@ -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) {
----------------
jansvoboda11 wrote:

Agreed. I'll go over this file and probably turn all nullable pointers into `std::optional<const X *>` (AFAIK these are only function return types and don't get stored in long-lived data structures) and assume plain `const X *` are not null.

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