Combination of commits

Fixes multiple issue with counters, with simplification

  Includes a change to the implicit else span in ast_lowering, so coverage
  of the implicit else no longer spans the `then` block.

  Adds coverage for unused closures and async function bodies.

  Fixes: #78542

Adding unreachable regions for known MIR missing from coverage map

Cleaned up PR commits, and removed link-dead-code requirement and tests

  Coverage no longer depends on Issue #76038 (`-C link-dead-code` is
  no longer needed or enforced, so MSVC can use the same tests as
  Linux and MacOS now)

Restrict adding unreachable regions to covered files

  Improved the code that adds coverage for uncalled functions (with MIR
  but not-codegenned) to avoid generating coverage in files not already
  included in the files with covered functions.

Resolved last known issue requiring --emit llvm-ir workaround

  Fixed bugs in how unreachable code spans were added.

compiler/rustc_codegen_ssa/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(bool_to_option)]
 #![feature(option_expect_none)]
 #![feature(box_patterns)]
+#![feature(drain_filter)]
 #![feature(try_blocks)]
 #![feature(in_band_lifetimes)]
 #![feature(nll)]