coverage: Add CoverageKind::SpanMarker for including extra spans in MIR

There are cases where coverage instrumentation wants to show a span for some
syntax element, but there is no MIR node that naturally carries that span, so
the instrumentor can't see it.

MIR building can now use this new kind of coverage statement to deliberately
include those spans in MIR, attached to a dummy statement that has no other
effect.

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -100,6 +100,9 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
 
         let Coverage { kind } = coverage;
         match *kind {
+            // Span markers are only meaningful during MIR instrumentation,
+            // and have no effect during codegen.
+            CoverageKind::SpanMarker => {}
             CoverageKind::CounterIncrement { id } => {
                 func_coverage.mark_counter_id_seen(id);
                 // We need to explicitly drop the `RefMut` before calling into `instrprof_increment`,