[Fixed Point Arithmetic] Fix for FixedPointValueToString

- Print negative numbers correctly
- Handle APInts of different sizes
- Add formal unit tests for FixedPointValueToString
- Add tests for checking correct printing when padding is set
- Restrict to printing in radix 10 since that's all we need for now

Differential Revision: https://reviews.llvm.org/D49945

llvm-svn: 339026