Use ASTContext::getDeclAlign(<Field Decl>) to get the alignment of the first

Use ASTContext::getDeclAlign(<Field Decl>) to get the alignment of the first
field to be memcpy'd, rather instead of ASTContext::getTypeAlign(<Field Type>).
For packed structs the alignment of a field may be less than the alignment of
the field's type.

<rdar://problem/13338585>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176512 91177308-0d34-0410-b5e6-96231b3b80d8