BitcodeReader: Require clients to read the block info block at most once.

BitcodeReader: Require clients to read the block info block at most once.

This change makes it the client's responsibility to call ReadBlockInfoBlock()
at most once. This is in preparation for a future change that will allow
there to be multiple block info blocks.

See also: http://lists.llvm.org/pipermail/llvm-dev/2016-October/106512.html

Differential Revision: https://reviews.llvm.org/D26016

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285350 91177308-0d34-0410-b5e6-96231b3b80d8