[Clang][BFloat16] Upgrade __bf16 by supporting increment/decrement operations

Since __bf16 has been upgraded from a storage-only type to an arithmetic type in https://reviews.llvm.org/rGe62175736551abf40a3410bc246f58e650eb8158, it should support all the basic arithmetic operations like other float types, including increment and decrement.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D152768