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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> > Ok, that's progress at least. I will then land the PR with the fix some time soon and we can take that item off the list.
> 
> How can we make sure the new reported errors are not triggered by #154158?

https://github.com/llvm/llvm-project/pull/154158 "relaxes" a check so that more conversion functions will compare equal (and subsequently be disambiguated). Also the fact that @emaxx-google mentions a "special build mode" (which probably involves `-fno-skip-odr-check-in-gmf`) makes me suspect that we're dealing with a different error here.

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