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_span/src/symbol.rs

@@ -2118,11 +2118,7 @@ impl Interner {
             return Symbol::new(idx as u32);
         }
 
-        // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
-        // and immediately convert the clone back to `&[u8]`, all because there
-        // is no `inner.arena.alloc_str()` method. This is clearly safe.
-        let string: &str =
-            unsafe { str::from_utf8_unchecked(inner.arena.alloc_slice(string.as_bytes())) };
+        let string: &str = inner.arena.alloc_str(string);
 
         // SAFETY: we can extend the arena allocation to `'static` because we
         // only access these while the arena is still alive.