Switch to APFloat constructor taking fltSemantics.

This change also makes the serialisation store the required semantics,
fixing an issue where PPC128 was always assumed when re-reading a
128-bit value.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173139 91177308-0d34-0410-b5e6-96231b3b80d8