coverage: Check that the fn sig span is before the body span

If the function signature span comes after the body span (due to macro
expansion), expanding it towards the start of the body span would produce
nonsense.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -330,6 +330,7 @@ fn make_coverage_hir_info(tcx: TyCtxt<'_>, def_id: LocalDefId) -> mir::coverage:
 
     let fn_sig_span = if let Some(fn_sig) = maybe_fn_sig
         && spans_are_compatible(fn_sig.span, body_span)
+        && fn_sig.span.lo() <= body_span.lo()
     {
         fn_sig.span.with_hi(body_span.lo())
     } else {