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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> 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.

I'm ok with pushing the commits one-by-one after the PR is reviewed, just let me know.

> > Complete only needed partial specializations: It is unclear (to me) why this needs to be done "for safety", but this change significantly improves the effectiveness of lazy loading.
> 
> This comes from the logic: if we have a partial template specialization `A<int, T, U>` and we need a full specialization for `A<int, double, double>`, we hope the partial specialization to be loaded

Sure, but in my understanding, that's not needed on the `ASTReader` side but is taken care of by `Sema` (?). For the following example:
```c++
//--- partial.cppm
export module partial;

export template <typename S, typename T, typename U>
struct Partial {
  static constexpr int Value() { return 0; }
};

export template <typename T, typename U>
struct Partial<int, T, U> {
  static constexpr int Value() { return 1; }
};

//--- partial.cpp
import partial;

static_assert(Partial<int, double, double>::Value() == 1);
```
(I assume that's what you have in mind?) I see two calls to `ASTReader::CompleteRedeclChain` (with this PR applied): The first asks for the full instantiation `Partial<int, double, double>` and regardless of what we load, the answer to the query is that it's not defined yet. The second asks for the partial specialization `Partial<int, T, U>` and then instantiation proceeds to do the right thing.

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