Unbox and unwrap the contents of StatementKind::Coverage

The payload of coverage statements was historically a structure with several
fields, so it was boxed to avoid bloating `StatementKind`.

Now that the payload is a single relatively-small enum, we can replace
`Box<Coverage>` with just `CoverageKind`.

This patch also adds a size assertion for `StatementKind`, to avoid
accidentally bloating it in the future.

src/coverageinfo.rs

@@ -1,11 +1,11 @@
 use rustc_codegen_ssa::traits::CoverageInfoBuilderMethods;
-use rustc_middle::mir::Coverage;
+use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::Instance;
 
 use crate::builder::Builder;
 
 impl<'a, 'gcc, 'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
-    fn add_coverage(&mut self, _instance: Instance<'tcx>, _coverage: &Coverage) {
+    fn add_coverage(&mut self, _instance: Instance<'tcx>, _kind: &CoverageKind) {
         // TODO(antoyo)
     }
 }