Rollup merge of #124308 - CBSpeir:diagnostic-item-enumerate, r=compil…

…er-errors

Add diagnostic item for `std::iter::Enumerate`

This adds a diagnostic item for `std::iter::Enumerate`.  The change will be used by the clippy `unused_enumerate_index` lint to move away from type paths to using diagnostic items.

see: rust-lang/rust-clippy#5393