[llvm] [clang] [clang][CoverageMapping] Refactor when setting MC/DC True/False (PR #78202)

[Jessica Paquette via cfe-commits]

================
@@ -722,6 +709,12 @@ struct MCDCCoverageBuilder {
       return I->second;
   }
 
+  /// Return the LHS Decision ({0,0} if not set).
+  const DecisionIDPair &back() {
+    assert(DecisionStack.size() >= 1);
----------------
ornata wrote:

This assert should be unnecessary. `SmallVector` already asserts for you:

```
  reference back() {
    assert(!empty());
    return end()[-1];
  }
  const_reference back() const {
    assert(!empty());
    return end()[-1];
  }
```

https://github.com/llvm/llvm-project/pull/78202