Tools, tests, and experimenting with MIR-derived coverage counters

Adds a new mir_dump output file in HTML/CSS to visualize code regions
and the MIR features that they came from (including overlapping spans).
See example below:

Includes a basic, MIR-block-based implementation of coverage injection,
available via `-Zexperimental-coverage`. This implementation has known
flaws and omissions, but is simple enough to validate the new tools and
tests.

The existing `-Zinstrument-coverage` option currently enables
function-level coverage only, which at least appears to generate
accurate coverage reports at that level.

Experimental coverage is not accurate at this time. When branch coverage
works as intended, the `-Zexperimental-coverage` option should be
removed.

This PR replaces the bulk of PR rust-lang#75828, with the remaining parts of
that PR distributed among other separate and indentpent PRs.

This PR depends on three of those other PRs: rust-lang#76000, rust-lang#76002, and

Rust compiler MCP rust-lang/compiler-team#278

Relevant issue: rust-lang#34701 - Implement support for LLVMs code coverage
instrumentation

![Screen-Recording-2020-08-21-at-2](https://user-images.githubusercontent.com/3827298/90972923-ff417880-e4d1-11ea-92bb-8713c6198f6d.gif)

compiler/rustc_middle/src/mir/mod.rs

@@ -1530,7 +1530,24 @@ impl Debug for Statement<'_> {
             AscribeUserType(box (ref place, ref c_ty), ref variance) => {
                 write!(fmt, "AscribeUserType({:?}, {:?}, {:?})", place, variance, c_ty)
             }
-            Coverage(box ref coverage) => write!(fmt, "{:?}", coverage),
+            Coverage(box ref coverage) => {
+                let rgn = &coverage.code_region;
+                match coverage.kind {
+                    CoverageKind::Counter { id, .. } => {
+                        write!(fmt, "Coverage::Counter({:?}) for {:?}", id.index(), rgn)
+                    }
+                    CoverageKind::Expression { id, lhs, op, rhs } => write!(
+                        fmt,
+                        "Coverage::Expression({:?}) = {} {} {} for {:?}",
+                        id.index(),
+                        lhs.index(),
+                        if op == coverage::Op::Add { "+" } else { "-" },
+                        rhs.index(),
+                        rgn
+                    ),
+                    CoverageKind::Unreachable => write!(fmt, "Coverage::Unreachable for {:?}", rgn),
+                }
+            }
             Nop => write!(fmt, "nop"),
         }
     }