[ScopInfo] Use MemoryAccess::getScopArrayInfo() interface to access Array [NFC]

By using the public interface MemoryAccess::getScopArrayInfo() we avoid the
direct access to the ScopArrayInfoMap and as a result also do not need to
use the BasePtr as key. This change makes the code cleaner.

The const-cast we introduce is a little ugly. We may consider to drop const
correctness for getScopArrayInfo() at some point.

This change removes unnecessary uses of MemoryAddress::getBaseAddr() in
preparation for https://reviews.llvm.org/D28518.

llvm-svn: 294655