[mlir] Add support for TF32 as a Builtin FloatType

This diff adds support for TF32 as a Builtin floating point type. This
supplements the recent addition of the TF32 semantic to the LLVM APFloat class
by extending usage to MLIR.

https://reviews.llvm.org/D151923

More information on the TF32 type can be found here:

https://blogs.nvidia.com/blog/2020/05/14/tensorfloat-32-precision-format/

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D153705