[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

mlir/test/python/ir/builtin_types.py

@@ -212,6 +212,8 @@ def testFloatType():
         print("float:", BF16Type.get())
         # CHECK: float: f16
         print("float:", F16Type.get())
+        # CHECK: float: tf32
+        print("float:", FloatTF32Type.get())
         # CHECK: float: f32
         print("float:", F32Type.get())
         # CHECK: float: f64