Fix use after free in BinaryStream library.

This was reported by the ASAN bot, and it turned out to be
a fairly fundamental problem with the design of VarStreamArray
and the way it passes context information to the extractor.

The fix was cumbersome, and I'm not entirely pleased with it,
so I plan to revisit this design in the future when I'm not
pressed to get the bots green again.  For now, this fixes
the issue by storing the context information by value instead
of by reference, and introduces some impossibly-confusing
template magic to make things "work".

llvm-svn: 301999