[clang] [coverage] fix crash in code coverage and `if constexpr` with `ExprWithCleanups` (PR #80292)

[via cfe-commits]

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@@ -1808,12 +1808,24 @@ struct CounterCoverageMappingBuilder
     }
   }
 
+private:
+  static bool evaluateConstantCondition(const Expr *Condition) {
+    if (const auto *Expr = dyn_cast<ConstantExpr>(Condition))
+      return Expr->getResultAsAPSInt().getExtValue();
+
+    if (const auto *Expr = dyn_cast<ExprWithCleanups>(Condition))
+      return evaluateConstantCondition(Expr->getSubExpr()); // recursion
+
+    assert(false && "Unexpected node in 'if constexpr' condition");
+    return false;
+  }
+
+public:
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cor3ntin wrote:

@AaronBallman This is basically an `IgnoreImpCast` except we want to stop at the first ConstantExpr (which is a FullExpr) - do you think this is worth adding a way to do that In `Expr`? 

https://github.com/llvm/llvm-project/pull/80292