Rollup merge of #160504 - lcnr:borrowck-cleanup, r=oli-obk

cleanup borrowck, improve c-variadic handling

The first commits of #160491. Hopefully all of them make sense.

It feels intuitive to me that the `c-variadic` region should be just another late-bound region and tracking region correctly for #160491 is otherwise a mess.

r? types

Author: JonathanBrouwer