Teach APFloat how to create both QNaNs and SNaNs and with arbitrary-width

Teach APFloat how to create both QNaNs and SNaNs and with arbitrary-width
payloads.  APFloat's internal folding routines always make QNaNs now,
instead of sometimes making QNaNs and sometimes SNaNs depending on the
type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97364 91177308-0d34-0410-b5e6-96231b3b80d8