Fix coverage ICE because fn_sig can have a span that crosses file boundaries

Fixes: #83792

MIR `InstrumentCoverage` assumed the `FnSig` span was contained within a
single file, but this is not always the case. Some macro constructions
can result in a span that starts in one `SourceFile` and ends in a
different one.

The `FnSig` span is included in coverage results as long as that span is
in the same `SourceFile` and the same macro context, but by assuming the
`FnSig` span's `hi()` and `lo()` were in the same file, I took this for
granted, and checked only that the `FnSig` `hi()` was in the same
`SourceFile` as the `body_span`.

I actually drop the `hi()` though, and extend the `FnSig` span to the
`body_span.lo()`, so I really should have simply checked that the
`FnSig` span's `lo()` was in the `SourceFile` of the `body_span`.

compiler/rustc_mir/src/transform/coverage/mod.rs

@@ -112,7 +112,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
         let source_file = source_map.lookup_source_file(body_span.lo());
         let fn_sig_span = match some_fn_sig.filter(|fn_sig| {
             fn_sig.span.ctxt() == body_span.ctxt()
-                && Lrc::ptr_eq(&source_file, &source_map.lookup_source_file(fn_sig.span.hi()))
+                && Lrc::ptr_eq(&source_file, &source_map.lookup_source_file(fn_sig.span.lo()))
         }) {
             Some(fn_sig) => fn_sig.span.with_hi(body_span.lo()),
             None => body_span.shrink_to_lo(),