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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

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

That's very good news. I think we can further reduce these times. IIRC, we still deserialize declarations that we do not need. One of the places to look is the logic that kicks in when at module loading time:
https://github.com/llvm/llvm-project/blob/772173f54868eef6e1a4d40ab93b0ee6c04b1aca/clang/lib/Serialization/ASTReader.cpp#L3426

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

Ouch. If that's the only issue on your infrastructure that's probably not so bad.
> 
> 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