Teach llvm-bcanalyzer to use one stream's BLOCKINFO to read another stream.

This allows streams that only use BLOCKINFO for debugging purposes to omit
the block entirely. As long as another stream is available with the correct
BLOCKINFO, the first stream can still be analyzed and dumped.

As part of this commit, BitstreamReader gets a move constructor and move
assignment operator, as well as a takeBlockInfo method.

llvm-svn: 216826