Add Float8E4M3FN type to MLIR.

The paper https://arxiv.org/abs/2209.05433 introduces two new FP8 dtypes: E5M2 (called Float8E5M2 in LLVM) and E4M3 (called Float8E4M3FN in LLVM). Support for Float8E5M2 in APFloat and MLIR was added in https://reviews.llvm.org/D133823. Support for Float8E4M3FN in APFloat was added in https://reviews.llvm.org/D137760. This change adds Float8E4M3FN to MLIR as well.

There is an RFC for adding the FP8 dtypes here: https://discourse.llvm.org/t/rfc-add-apfloat-and-mlir-type-support-for-fp8-e5m2/65279.

This change is identical to the MLIR changes in the patch that added Float8E5M2, except that Float8E4M3FN is added instead.

Reviewed By: stellaraccident, bkramer, rriddle

Differential Revision: https://reviews.llvm.org/D138075