[clang] [Serialization] Fix lazy template loading (PR #133057)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

The small-scale benchmarks we had show 10% improvement in CPU and 23% improvement in memory usage for some compilations!

We did hit one compiler error that does not reproduce without modules, however:
```error: use of overloaded operator '=' is ambiguous```

We're in the process of getting a small reproducer (please bear with us, it takes some time) that we can share. @emaxx-google is working on it.

https://github.com/llvm/llvm-project/pull/133057