[APFloat] APFloat::Storage::Storage - refix use after move

Summary:
Re-land r360675 after it was reverted in r360770.

This was reported in:
https://llvm.org/reports/scan-build/

Based on feedback in:
https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190513/652286.html

Reviewers: RKSimon, efriedma

Reviewed By: RKSimon, efriedma

Subscribers: eli.friedman, hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62767

llvm-svn: 362833