Auto merge of #45525 - MaikKlein:collector, r=eddyb

Auto merge of #45525 - MaikKlein:collector, r=eddyb

Move collector to librustc_mir::monomorphize

cc https://github.com/rust-lang/rust/issues/44334 and https://github.com/rust-lang/rust/issues/45276

* I moved the collector to rustc_mir

*  I renamed `TransItem` to `MonoItem`. _(I still need to fix up comments and variable names)_

* I got rid of `common.rs` and `monomorphize.rs` from `librustc_trans_utils`. I moved most of the functionality into `TyCtxt`. I realized that the `librustc_trans_utils::common.rs` was just copy pasted from `librustc_trans::common.rs`.

Should I also get rid of the `librustc_trans::common.rs` in this PR? Most of the functionality seems a bit useless, I decided to put some of it into `TyCtxt` but maybe that is not the correct action here.

Should I also get rid of `librustc_trans_utils` completely here? Or should I do it in a separate PR?