Chris's constant data sequence refactoring actually enabled printing

Chris's constant data sequence refactoring actually enabled printing
vectors of all one bits to be printed more cleverly in the AsmPrinter.
Unfortunately, the byte value for all one bits is the same with
-fsigned-char as the error return of '-1'. Force this to be the unsigned
byte value when returning it to avoid this problem, and update the test
case for the shiny new behavior.

Yay for building LLVM and Clang with -funsigned-char.

Chris, please review, and let me know if there is any reason to not
desire this change. It seems good on the surface, and certainly intended
based on the code written.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149299 91177308-0d34-0410-b5e6-96231b3b80d8