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.

compiler/rustc_span/src/symbol.rs

@@ -323,6 +323,7 @@ symbols! {
         await_macro,
         bang,
         begin_panic,
+        begin_panic_fmt,
         bench,
         bin,
         bind_by_move_pattern_guards,
@@ -420,6 +421,7 @@ symbols! {
         const_loop,
         const_mut_refs,
         const_panic,
+        const_panic_fmt,
         const_precise_live_drops,
         const_ptr,
         const_raw_ptr_deref,
@@ -586,6 +588,7 @@ symbols! {
         fmaf32,
         fmaf64,
         fmt,
+        fmt_as_str,
         fmt_internals,
         fmul_fast,
         fn_align,
@@ -881,6 +884,7 @@ symbols! {
         panic_2021,
         panic_abort,
         panic_bounds_check,
+        panic_fmt,
         panic_handler,
         panic_impl,
         panic_implementation,