[clang] Fix -Wunused-variable in SemaChecking.cpp (NFC)

llvm-project/clang/lib/Sema/SemaChecking.cpp:19192:15:
error: unused variable 'Field1Parent' [-Werror,-Wunused-variable]
  const Type *Field1Parent = Field1->getParent()->getTypeForDecl();
              ^
llvm-project/clang/lib/Sema/SemaChecking.cpp:19193:15:
error: unused variable 'Field2Parent' [-Werror,-Wunused-variable]
  const Type *Field2Parent = Field2->getParent()->getTypeForDecl();
              ^
2 errors generated.