Rollup merge of #116007 - m-ou-se:const-panic-fmt-panic-display, r=oli-obk

Call panic_display directly in const_panic_fmt.

`panic_str` just directly calls `panic_display`. The only reason `panic_str` exists, is for a lint to detect an expansion of `panic_2015!(expr)` (which expands to `panic_str`).

It is `panic_display` that is hooked by const-eval, which is the reason we call it here.

Part of rust-lang/rust#116005

r? ``@oli-obk``

Author: GuillaumeGomez