Initial (incomplete) implementation of transmutability trait.

This initial implementation handles transmutations between types with specified layouts, except when references are involved.

Co-authored-by: Igor null <m1el.2027@gmail.com>

compiler/rustc_span/src/symbol.rs

@@ -1462,6 +1462,7 @@ symbols! {
         trait_alias,
         trait_upcasting,
         transmute,
+        transmute_trait,
         transparent,
         transparent_enums,
         transparent_unions,