[APFloat] Make functions that produce APFloaat objects use correct semantics.

Summary:
Fixes PR30869.

In D25977 I meant to change all functions that care about lifetime. I
changed constructors, factory functions, but I missed member/free
functions that return new instances. This patch changes them.

Reviewers: hfinkel, kbarton, echristo, joerg

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D26269

llvm-svn: 286060