Bitcode: Decouple block info block state from reader.

As proposed on llvm-dev:
http://lists.llvm.org/pipermail/llvm-dev/2016-October/106630.html

Move block info block state to a new class, BitstreamBlockInfo.
Clients may set the block info for a particular cursor with the
BitstreamCursor::setBlockInfo() method.

At this point BitstreamReader is not much more than a container for an
ArrayRef<uint8_t>, so remove it and replace all uses with direct uses
of memory buffers.

Differential Revision: https://reviews.llvm.org/D26259

llvm-svn: 286207