ast: unwrap MacroSource before `jl_timing_show_macro` (#61642)

When a macro accepts a `Core.MacroSource` argument (e.g. `@VERSION`) and
the first method lookup succeeds, `margs[1]` remains a `MacroSource`
rather than the inner `LineNumberNode`. Passing it to
`jl_timing_show_macro` then trips an assertion in assertions builds. Use
the already-unwrapped `retry_lno` for the timing call when available.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>