[IRBuilder] Update IRBuilder::CreateFNeg(...) to return a UnaryOperator

Reapply r374240 with fix for Ocaml test, namely Bindings/OCaml/core.ml.

Differential Revision: https://reviews.llvm.org/D61675

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374782 91177308-0d34-0410-b5e6-96231b3b80d8
20 files changed