Auto merge of #86998 - m-ou-se:const-panic-fmt-as-str, r=oli-obk

Make const panic!("..") work in Rust 2021.

During const eval, this replaces calls to core::panicking::panic_fmt and std::panicking::being_panic_fmt with a call to a new const fn: core::panicking::const_panic_fmt. That function uses fmt::Arguments::as_str() to get the str and calls panic_str with that instead.

panic!() invocations with formatting arguments are still not accepted, as the creation of such a fmt::Arguments cannot be done in constant functions right now.

r? `@RalfJung`