[MIR]-based translation of extension method calls does not actually work yet

[michaelwoerister]

The extension method test in run-pass/mir_trans_calls.rs does not actually test the MIR translation path because the #[rustc_mir] attribute on the function is not picked up.
There is already PR #30661 that fixes the ignoring of attributes on generic functions. Once it has landed, the test method can be used in its current form. Otherwise it would have to be rewritten in a non-generic way.