CodeGen: ConstStructBuilder must verify packed constraints after padding

Before, ConstStructBuilder::AppendBytes would check packed constraints
prior to padding being added before the field's offset.  However, adding
this padding might force our struct to be packed.  Because we wouldn't
check *after* adding padding, ConstStructBuilder would be in an
inconsistent state leading to a crash.

This fixes PR21300.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@220153 91177308-0d34-0410-b5e6-96231b3b80d8