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/decl_name_stack.cpp

@@ -168,7 +168,8 @@ auto DeclNameStack::ApplyNameQualifierTo(NameContext& name_context,
   if (TryResolveQualifier(name_context, loc_id)) {
     // For identifier nodes, we need to perform a lookup on the identifier.
     auto resolved_inst_id = context_->LookupNameInDecl(
-        name_context.loc_id, name_id, name_context.target_scope_id);
+        name_context.loc_id, name_id, name_context.target_scope_id,
+        /*mark_imports_used=*/false);
     if (!resolved_inst_id.is_valid()) {
       // Invalid indicates an unresolved name. Store it and return.
       name_context.state = NameContext::State::Unresolved;