Make the assertion in Ident::new debug-only.

This fixes a perf regression introduced in #140252.

(cherry picked from commit 4cb9f0309d)

compiler/rustc_span/src/symbol.rs

@@ -2353,7 +2353,7 @@ impl Ident {
     #[inline]
     /// Constructs a new identifier from a symbol and a span.
     pub fn new(name: Symbol, span: Span) -> Ident {
-        assert_ne!(name, kw::Empty);
+        debug_assert_ne!(name, kw::Empty);
         Ident { name, span }
     }