Fix evaluateConstantInt (immunant#339)

* Fix evaluateConstantInt

Ensure we set the value when there is one, instead of when there isn't.

* Use getIntegerConstantExpr in evaluateConstantInt

isIntegerConstantExpr no longer retrieves the constant from an expr in
LLVM 12, so we need to call our helper to do that to preserve the old
behavior.

Co-authored-by: Stephen Crane <[email protected]>

Author: mjgarton

c2rust-ast-exporter/src/AstExporter.cpp

@@ -484,22 +484,25 @@ class TranslateASTVisitor final
     }
 
     bool evaluateConstantInt(Expr *E, APSInt &constant) {
-        bool hasValue = E->isIntegerConstantExpr(*Context);
+        auto value = getIntegerConstantExpr(*E, *Context);
 
-        if (!hasValue) {
+        if (value) {
+            constant = *value;
+            return true;
+        } else {
 #if CLANG_VERSION_MAJOR < 8
             APSInt eval_result;
 #else
             Expr::EvalResult eval_result;
 #endif // CLANG_VERSION_MAJOR
-            hasValue = E->EvaluateAsInt(eval_result, *Context);
+            bool hasValue = E->EvaluateAsInt(eval_result, *Context);
 #if CLANG_VERSION_MAJOR < 8
             constant = eval_result;
 #else
             constant = eval_result.Val.getInt();
 #endif // CLANG_VERSION_MAJOR
+            return hasValue;
         }
-        return hasValue;
     }
 
     // Template required because Decl and Stmt don't share a common base class