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.

compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs

@@ -1,5 +1,5 @@
 use super::BackendTypes;
-use rustc_middle::mir::Coverage;
+use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::Instance;
 
 pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
@@ -7,5 +7,5 @@ pub trait CoverageInfoBuilderMethods<'tcx>: BackendTypes {
     ///
     /// This can potentially be a no-op in backends that don't support
     /// coverage instrumentation.
-    fn add_coverage(&mut self, instance: Instance<'tcx>, coverage: &Coverage);
+    fn add_coverage(&mut self, instance: Instance<'tcx>, kind: &CoverageKind);
 }