Auto merge of #118189 - compiler-errors:cache-flags-for-const, r=nnethercote

Cache flags for `ty::Const`

Not sure if this has been attempted yet, but worth a shot. It does make the code simpler in `rustc_type_ir`, since we can assume that consts have a `flags` method that is no-cost.

r? `@ghost`

Author: bors