Bitcode: Split out SimpleBitstreamCursor

Split out SimpleBitstreamCursor from BitstreamCursor, which is a
lower-level cursor with no knowledge of bitcode blocks, abbreviations,
or records.  It just knows how to read bits and navigate the stream.

This is mainly organizational, to separate the API for manipulating raw
bits from that for bitcode concepts like Record and Block.

llvm-svn: 264545