perf: remove unnecessary string clone in maybe_concat_string_literal

[andygrove]

Summary

Remove an unnecessary String::clone() in maybe_concat_string_literal().

Changes

// Before:
str.push_str(s.clone().as_str());

// After:
str.push_str(s);

Since s is already a &String (from the ref s pattern match), it can be passed directly to push_str() which accepts &str. The deref coercion handles the conversion automatically.

Impact

This removes one heap allocation per concatenated string literal when parsing SQL with adjacent string literals like:

SELECT 'foo' 'bar'  -- Concatenates to 'foobar'

Small change but in the spirit of avoiding unnecessary allocations in the parser hot path.

Test plan

• All existing tests pass
• cargo clippy passes

🤖 Generated with Claude Code

[iffyio]

LGTM! Thanks @andygrove!