[FEnv] File with properties of constrained intrinsics

Summary
In several places we need to enumerate all constrained intrinsics or IR
nodes that should be represented by them. It is easy to miss some of
the cases. To make working with these intrinsics more convenient and
robust, this change introduces file containing definitions of all
constrained intrinsics and some of their properties. This file can be
included to generate constrained intrinsics processing code.

Reviewers: kpn, andrew.w.kaylor, cameron.mcinally, uweigand

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69887