Rollup merge of #155358 - niacdoial:improperctypes-refactor2.1, r=petrochenkov

ImproperCTypes: Move erasing_region_normalisation into helper function

This is "part 1/3 of 2/3 of 1/2" of the original pull request rust-lang/rust#134697 (refactor plus overhaul of the ImproperCTypes family of lints)
(all pulls of this series of pulls are supersets of the previous pulls. If this pull is "too small" to be worth the effort, you can instead look at the next in the series)

This pull is a small internal change among the efforts to refactor the ImproperCTypes lints, by moving some "unwrapping" code (`cx.tcx.try_normalize_erasing_regions`) into a helper function.

r? petrochenkov

Author: Zalathar