[mlir] Use double format when parsing bfloat16 hexadecimal values

Summary: bfloat16 doesn't have a valid APFloat format, so we have to use double semantics when storing it. This change makes sure that hexadecimal values can be round-tripped properly given this fact.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D72667