Define a `cppAccessorType` to const-ref in APFloatParameter and update ODS...

Define a `cppAccessorType` to const-ref in APFloatParameter and update ODS emitter to use it for verifier signatures

This reduce an unnecessary amount of copy of non-trivial objects, like
APFloat.

Reviewed By: rriddle, jpienaar

Differential Revision: https://reviews.llvm.org/D116505