[clang][RISCV] Fix ABI lowering for _Float16 for FP ABIs

For trivial cases (`_Float16` as a standalone argument), it was
previously correctly lowered to half. But the logic for catching cases
involving structs was gated off, as at the time that logic was written
the ABI for half was unclear.

This patch fixes that and adds a release note.

Differential Revision: https://reviews.llvm.org/D145074