[APFloat] Reduce some dispatch boilerplates. NFC.

Summary: This is an attempt to reduce the verbose manual dispatching code in APFloat. This doesn't handle multiple dispatch on single discriminator (e.g. APFloat::add(const APFloat&)), nor handles multiple dispatch on multiple discriminators (e.g. APFloat::convert()).

Reviewers: hfinkel, echristo, jlebar

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D29161

llvm-svn: 293255