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