[CodeGen] Use IRBuilder::CreateFNeg for __builtin_conj

This replaces the fsub -0.0 idiom with an fneg instruction. We didn't see to have a test that showed the current codegen. Just some tests for constant folding and a test that was only checking the declare lines for libcalls. The latter just checked that we did not have a declare for @conj when using __builtin_conj.

Differential Revision: https://reviews.llvm.org/D72012