[BinaryStream] Support growable streams.

[BinaryStream] Support growable streams.

The existing library assumed that a stream's length would never
change.  This makes some things simpler, but it's not flexible
enough for what we need, especially for writable streams where
what you really want is for each call to write to actually append.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319070 91177308-0d34-0410-b5e6-96231b3b80d8