[Fixed Point Arithmetic] Rename `-fsame-fbits` flag

- Rename the `-fsame-fbits` flag to `-fpadding-on-unsigned-fixed-point`
- Move the flag from a driver option to a cc1 option
- Rename the `SameFBits` member in TargetInfo to `PaddingOnUnsignedFixedPoint`
- Updated descriptions

Differential Revision: https://reviews.llvm.org/D48727

llvm-svn: 335993