Rollup merge of #150025 - BoxyUwU:mgca_no_unused_defids, r=oli-obk

dont create unnecessary `DefId`s under mgca

Fixes rust-lang/rust#149977
Fixes rust-lang/rust#148838

Accidentally left this out of rust-lang/rust#149136 even though being able to do this was a large part of the point of the PR :3

First ICE was caused by the fact that we create a defid but never lower the nodeid associated with it to a hirid which later parts of the compiler can't handle.

See test for second ICE

r? oli-obk

Author: JonathanBrouwer