APFloat: allow 64-bit of payload

Summary: The APFloat and Constant APIs taking an APInt allow arbitrary payloads,
and that's great. There's a convenience API which takes an unsigned, and that's
silly because it then directly creates a 64-bit APInt. Just change it to 64-bits
directly.

At the same time, add ConstantFP NaN getters which match the APFloat ones (with
getQNaN / getSNaN and APInt parameters).

Improve the APFloat testing to set more payload bits.

Reviewers: scanon, rjmccall

Subscribers: jkorous, dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D55460

llvm-svn: 348791