Improved the MIR spanview output

* Adds missing "tail" spans (spans that continue beyond the end of
overlapping spans)
* Adds a caret to highlight empty spans associated with MIR elements
that have a position, but otherwise would not be visible.
* Adds visual pointing brackets at the beginning and end of each span

compiler/rustc_mir/src/transform/instrument_coverage.rs

@@ -309,7 +309,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
         for coverage_region in coverage_regions {
             span_viewables.push(SpanViewable {
                 span: coverage_region.span,
-                title: format!("{}", coverage_region.blocks[0].index()),
+                id: format!("{}", coverage_region.blocks[0].index()),
                 tooltip: self.make_tooltip_text(coverage_region),
             });
         }