Auto merge of #140466 - amandasystems:move-to-preprocessing-step, r=lcnr

Move placeholder handling to a proper preprocessing step

This commit breaks out the logic of placheolder rewriting into its own preprocessing step. It's one of the more boring
parts of #130227.

The only functional change from this is that the preprocessing step (where extra `r: 'static` constraints are added) is performed upstream of Polonius legacy, finally affecting Polonius. That is mostly a by-product, though.

This should be reviewable by anyone in the compiler team, so
r? rust-lang/compiler

Author: bors