Fix testing for end of stream in bitstream reader.

This fixes a bug found while working on the bitcode reader. In
particular, the method BitstreamReader::AtEndOfStream doesn't always
behave correctly when processing a data streamer. The method
fillCurWord doesn't properly set CurWord/BitsInCurWord if the data
streamer was already at eof, but GetBytes had not yet set the
ObjectSize field of the streaming memory object.

This patch fixes this problem, and provides a test to show that
this problem has been fixed.

Patch by Karl Schimpf.

Differential Revision: http://reviews.llvm.org/D11391

llvm-svn: 243890