Auto merge of #123071 - rcvalle:rust-cfi-fix-method-fn-ptr-cast, r=compiler-errors

CFI: Fix methods as function pointer cast

Fix casting between methods and function pointers by assigning a secondary type id to methods with their concrete self so they can be used as function pointers.

This was split off from #116404.

cc `@compiler-errors` `@workingjubilee`

Author: bors