`FindInstantiatedDecl()` relies on the `CurContext` to find the
corresponding class template instantiation for a class template
declaration.
Previously, we pushed the semantic declaration context for constraint
comparison, which is incorrect for constraints on friend declarations.
In issue #78101, the semantic context of the friend is the TU, so we
missed the implicit template specialization `Template<void, 4>` when
looking for the instantiation of the primary template `Template` at the
time of checking the member instantiation; instead, we mistakenly picked
up the explicit specialization `Template<float, 5>`, hence the error.
As a bonus, this also fixes a crash when diagnosing constraints. The
DeclarationName is not necessarily an identifier, so it's incorrect to
call `getName()` on e.g. overloaded operators. Since the
DiagnosticBuilder has correctly handled Decl printing, we don't need to
find the printable name ourselves.
Fixes https://github.com/llvm/llvm-project/issues/78101
358165ded3
