coverage: Store all of a function's mappings in function coverage info

Previously, mappings were attached to individual coverage statements in MIR.
That necessitated special handling in MIR optimizations to avoid deleting those
statements, since otherwise codegen would be unable to reassemble the original
list of mappings.

With this change, a function's list of mappings is now attached to its MIR
body, and survives intact even if individual statements are deleted by
optimizations.

compiler/rustc_middle/src/query/mod.rs

@@ -581,17 +581,6 @@ rustc_queries! {
         arena_cache
     }
 
-    /// Returns the `CodeRegions` for a function that has instrumented coverage, in case the
-    /// function was optimized out before codegen, and before being added to the Coverage Map.
-    query covered_code_regions(key: DefId) -> &'tcx Vec<&'tcx mir::coverage::CodeRegion> {
-        desc {
-            |tcx| "retrieving the covered `CodeRegion`s, if instrumented, for `{}`",
-            tcx.def_path_str(key)
-        }
-        arena_cache
-        cache_on_disk_if { key.is_local() }
-    }
-
     /// The `DefId` is the `DefId` of the containing MIR body. Promoteds do not have their own
     /// `DefId`. This function returns all promoteds in the specified body. The body references
     /// promoteds by the `DefId` and the `mir::Promoted` index. This is necessary, because