When merging functions, detect uses of imports prior to a redecl. (#3836)

LookupNameInDecl is only called from DeclNameStack, but I'm adding
mark_imports_used there because it feels more consistent. Not sure if we
want a better API boundary. I admit I'm also suspicious of its call to
LookupInCurrentScope but maybe it's okay due to how imports work.

I was choosing to print multiple diagnostics when a declaration is
previously used _and_ doesn't match because I think the "previously
used" is more important, but the "doesn't match" may give an additional
hint about why it didn't work.

The merge.h utility function is because I think we can follow a similar
model for identifying errors with other declarations: classes,
interfaces, etc.

toolchain/check/impl.cpp

@@ -92,8 +92,8 @@ static auto BuildInterfaceWitness(
     auto decl = context.insts().Get(const_id.inst_id());
     if (auto fn_decl = decl.TryAs<SemIR::FunctionDecl>()) {
       auto& fn = context.functions().Get(fn_decl->function_id);
-      auto impl_decl_id =
-          context.LookupNameInExactScope(decl_id, fn.name_id, impl_scope);
+      auto impl_decl_id = context.LookupNameInExactScope(
+          decl_id, fn.name_id, impl_scope, /*mark_imports_used=*/true);
       if (impl_decl_id.is_valid()) {
         used_decl_ids.push_back(impl_decl_id);
         table.push_back(CheckAssociatedFunctionImplementation(