Remove special case for returning value expressions by copy (#6052)

When returning a value from a function whose return type has a by-copy
initializing representation, perform initialization like we do when the
return type has an in-place initializing representation. This makes our
SemIR representation more uniform, as the return expression will now
always be an initializing expression rather than a value expression, but
more importantly it means that attempts to return a non-copyable type by
value now fail, even if the type has a by-copy initializing
representation.

This catches a bunch of places where we were returning a value of an
unconstrained template parameter `T:! type`, which we were incorrectly
allowing because we didn't notice it was not copyable. Unfortunately
this then requires quite a few test updates.

Like #6034, this exposes a lowering issue where lowering crashes when
attempting to lower a specific copy operation for certain types; a
couple more tests are temporarily disabled here. An upcoming PR
dependent on this one will fix the issue and re-enable those tests.

toolchain/check/return.cpp

@@ -157,13 +157,10 @@ auto BuildReturnWithExpr(Context& context, SemIR::LocId loc_id,
     // We already diagnosed that the return type is invalid. Don't try to
     // convert to it.
     expr_id = SemIR::ErrorInst::InstId;
-  } else if (return_info.has_return_slot()) {
+  } else {
     return_slot_id = GetCurrentReturnSlot(context);
     CARBON_CHECK(return_slot_id.has_value());
     expr_id = Initialize(context, loc_id, return_slot_id, expr_id);
-  } else {
-    expr_id =
-        ConvertToValueOfType(context, loc_id, expr_id, return_info.type_id);
   }
 
   AddReturnCleanupBlockWithExpr(