Explicitly test BitstreamReader::setArtificialByteLimit, NFC

Explicitly check that artificial byte limit is rounded correctly by
exposing BitstreamReader::Size through a new accessor, getSizeIfKnown.

The original code for rounding (from r264547) wasn't obviously correct,
and even though r264623 cleaned it up (by calling llvm::alignTo) I think
it's worth testing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264650 91177308-0d34-0410-b5e6-96231b3b80d8