[Clang] Instantiate the correct lambda call operator (#110446)

This is a fix for the following issue: when a lambda’s class type is
merged across modules (e.g. because it is defined in a template in the
GMF of some module `A`, and some other module `B` both imports `A` and
has the same template in its GMF), then `getLambdaCallOperator()` might
return the wrong operator (e.g. while compiling `B`, the lambda’s class
type would be the one attached to `B`’s GMF, but the call operator ends
up being the one attached to `A`’s GMF).

This causes issues in situations where the call operator is in a
template and accesses declarations in the surrounding context: when
those declarations are instantated, a mapping is introduced from the
original node in the template to that of the instantiation. If such an
instantiation happens in `B`, and we then try to instantiate `A`’s call
operator, any nodes in that call operator refer to declarations in the
template in `A`, but the `LocalInstantiationScope` only contains
mappings for declarations in `B`! This causes the following assertion
(for godbolt links and more, see the issue below):

```
Assertion `isa<LabelDecl>(D) && "declaration not instantiated in this scope"' failed.
```

We now walk the redecl chain of the call operator to find the
one that is in the same module as the record decl.

This fixes #110401.

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