[RISCV] Emit .variant_cc directives for vector function calls.

[RISCV] Emit .variant_cc directives for vector function calls.

The patch is splitted from D103435. The patch emits .variant_cc [0] for those
function calls that have vector arguments or vector return values.

[0]: https://github.com/riscv/riscv-elf-psabi-doc/pull/190

Initial authored by: HsiangKai

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D139414