[PDB] Add an additional test for BinaryStreamRef.

A bug was uncovered where if you have a StreamRef whose ViewOffset
is > 0, then when you call readLongestContiguousChunk it will
succeed even when it shouldn't, and it always return you a
buffer that was taken as if the ViewOffset was 0.

Fixed this bug and added a test for it.

llvm-svn: 296556