[clang] Fix a crash in constant evaluation

This was showing up in our internal crash collector. I have no idea how
to test it out though, open for suggestions if there are easy paths but
otherwise I'd move forward with the patch.

Differential Revision: https://reviews.llvm.org/D132918

Author: kadircet

clang/lib/AST/ExprConstant.cpp

@@ -4794,6 +4794,11 @@ static bool getDefaultInitValue(QualType T, APValue &Result) {
       Result = APValue((const FieldDecl *)nullptr);
       return true;
     }
+    // Can't access properties of an incomplete type.
+    if (!RD->hasDefinition()) {
+      Result = APValue();
+      return false;
+    }
     Result = APValue(APValue::UninitStruct(), RD->getNumBases(),
                      std::distance(RD->field_begin(), RD->field_end()));