Return the number of read bytes in MemoryObject::readBytes.

Returning more information will allow BitstreamReader to be simplified a bit
and changed to read 64 bits at a time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221794 91177308-0d34-0410-b5e6-96231b3b80d8