Get rid of IMsfStreamData class.

This was a pure virtual base class whose purpose was to abstract
away the notion of how you retrieve the layout of a discontiguous
stream of blocks in an Msf file.  This led to too many layers of
abstraction making it difficult to figure out what was going on
and extend things.  Ultimately, a stream's layout is decided by
its length and the array of block numbers that it lives on.  So
rather than have an abstract base class which can return this in
any number of ways, it's more straightforward to simply store them
as fields of a trivial struct, and also to give a more appropriate
name.

This patch does that.  It renames IMsfStreamData to MsfStreamLayout,
and deletes the 2 concrete implementations, DirectoryStreamData
and IndexedStreamData.  MsfStreamLayout is a trivial struct
with the necessary data.

llvm-svn: 277018