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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I can confirm that we routinely rely on the same header being part of multiple modules and results of that being combined in all kinds of ways imaginable. Moreover, the same header can be a modular header in some modules, textual in others and the resulting PCMs may also be combined together later (i.e. used as a direct or transitive dependencies of another compilation).

Clang currently supports that.

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