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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> @ilya-biryukov, would you mind giving this PR a test on your infrastructure and if it works maybe share some performance results?

Sure, let me try kicking it off. Note that our infrastructure is much better at detecting the compilations timing out than providing proper benchmarking at scale (there are a few targeted benchmarks too, though).
That means we're good and detecting big regressions, but won't be able to provide very reliable performance measurements.

I'll try to give you what we have, though.

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