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

[Jan Svoboda via llvm-commits]

================
@@ -226,9 +247,28 @@ class DependencyScanningFilesystemLocalCache {
   insertEntryForFilename(StringRef Filename,
                          const CachedFileSystemEntry &Entry) {
     assert(llvm::sys::path::is_absolute_gnu(Filename));
-    const auto *InsertedEntry = Cache.insert({Filename, &Entry}).first->second;
-    assert(InsertedEntry == &Entry && "entry already present");
-    return *InsertedEntry;
+    assert(Cache[Filename].first == nullptr && "entry already present");
+    Cache[Filename].first = &Entry;
+    return Entry;
----------------
jansvoboda11 wrote:

The complication is that the cache entry may now already exist, but will only have the real path populated. I guess we can get close to the existing behavior with something like this:

```
    auto [It, Inserted] = Cache.insert({Filename, {&Entry, nullptr}});
    auto &[CachedEntry, CachedRealPath] = It->getValue();
    if (!Inserted) {
      // The file is already present in the local cache. If we got here, it only
      // contains the real path. Let's make sure the entry is populated too.
      assert((!CachedEntry && CachedRealPath) && "entry already present");
      CachedEntry = &Entry;
    }
    return *CachedEntry;
```

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