Add codegen for llvm exp/exp2 elementwise builtins

Add codegen for llvm exp/exp2 elementwise builtin
The exp/exp2 elementwise builtins are necessary for HLSL codegen.
Tests were added to make sure that the expected errors are encountered when these functions are given inputs of incompatible types.
The new builtins are restricted to floating point types only.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D145270