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

[Maksim Ivanov via cfe-commits]

emaxx-google wrote:

Understood. JFYI I've been running another minimization task, this time without the `-fallow-pcm-with-compiler-errors` parameter, so that all modules compile correctly and the result is easier to comprehend, but it obviously takes a lot longer to reduce - so far it's still at 100 MB after running for 3 days (the start size was 0.5 GB).

We do have tools to filter out unrelated files, still the initial input size is that big. But we continue looking into various aspects of the minimization performance.

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