raw_ostream: Reduce FormattedStream's reliance on raw_ostream's implementation.

 - Kill off begin(), end(), and iterator. It isn't clear what these
   mean. Instead provide getBufferStart(), which can be used with
   GetNumBytesInBuffer to the same effect.

 - Update ComputeColumn to take arguments for the buffer to scan, this
   simplifies the implementation of write_impl substantially.

 - This should also fix possible problems with the scanning pointer pointing
   outside of the current raw_ostream buffer.

llvm-svn: 79379