Rollup merge of rust-lang#76313 - richkadel:mir-spanview-2, r=wesleywiser

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

<img width="590" alt="Screen Shot 2020-09-03 at 8 38 08 PM" src="https://user-images.githubusercontent.com/3827298/92202571-25510c00-ee34-11ea-89bc-89eea939476d.png">
<img width="1061" alt="Screen Shot 2020-09-03 at 8 41 04 PM" src="https://user-images.githubusercontent.com/3827298/92202629-49145200-ee34-11ea-8fda-fc6e62c80736.png">
<img width="1113" alt="Screen Shot 2020-09-06 at 5 42 57 PM" src="https://user-images.githubusercontent.com/3827298/92339198-ca085f00-f069-11ea-96d1-c01ced50e2ba.png">
<img width="1692" alt="Screen Shot 2020-09-06 at 5 45 54 PM" src="https://user-images.githubusercontent.com/3827298/92339209-d4c2f400-f069-11ea-94c0-b4d36c200878.png">

r? @tmandry
FYI: @wesleywiser

Author: tmandry

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),
             });
         }