[MLIR] Add function to create BFloat16 array attribute

This patch adds a new function `mlirDenseElementsAttrBFloat16Get()`,
which accepts the shaped type, the number of BFloat16 values, and a
pointer to an array of BFloat16 values, each of which is a `uint16_t`
value.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D123981