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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> > > While I may not able to look into them in detail recently, it may be helpful to split this into seperate patches to review and to land.
> > 
> > 
> > I initially considered this, but @vgvassilev said in [root-project/root#17722 (comment)](https://github.com/root-project/root/pull/17722#issuecomment-2706555950) he prefers a single PR, also for external testing.
> 
> Maybe you can test it with this and land it with different patches. So that we can revert one of them if either of them are problematic but other parts are fine.

This is a relatively small patch focused on reducing the round trips to modules deserialization. I see this as an atomic change that if it goes in partially would defeat its purpose. What's the goal of a partial optimization?

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