perf: remove unnecessary string clone in maybe_concat_string_literal

Replace `s.clone().as_str()` with just `s` since `s` is already a
`&String` which can be passed directly to `push_str()`.

This removes one String allocation per concatenated string literal
when parsing SQL with adjacent string literals like 'foo' 'bar'.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: andygrove

src/parser/mod.rs

@@ -11299,7 +11299,7 @@ impl<'a> Parser<'a> {
             while let Token::SingleQuotedString(ref s) | Token::DoubleQuotedString(ref s) =
                 self.peek_token_ref().token
             {
-                str.push_str(s.clone().as_str());
+                str.push_str(s);
                 self.advance_token();
             }
         }