rustc_arena: add alloc_str

Two places called `from_utf8_unchecked` for strings from `alloc_slice`,
and one's SAFETY comment said this was for lack of `alloc_str` -- so
let's just add that instead!

compiler/rustc_middle/src/ty/mod.rs

@@ -2614,9 +2614,7 @@ pub struct SymbolName<'tcx> {
 
 impl<'tcx> SymbolName<'tcx> {
     pub fn new(tcx: TyCtxt<'tcx>, name: &str) -> SymbolName<'tcx> {
-        SymbolName {
-            name: unsafe { str::from_utf8_unchecked(tcx.arena.alloc_slice(name.as_bytes())) },
-        }
+        SymbolName { name: tcx.arena.alloc_str(name) }
     }
 }