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

[fakepaper56]

Summary

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

[kito-cheng]

LGTM from the psABI perspective, but I would like to wait another week to make sure other review is happy :)

llvm/test/CodeGen/RISCV/rvv/variant-cc.ll

14–19

I am surprise VLS type will set .variant_cc too, but I realized soon that's kind of LLVM ABI extensions.

[smd]

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

233

Hi,
Could this label printing be extended not only for vector calls, but also for functions that has variant_cc flag set?
I guess this is what I need for D132994 to go further.
Thanks

[jrtc27]

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

233

Your code should just be calling emitDirectiveVariantCC, not poking around in the symbol directly. Emitting the directive based on st_other is backwards.

[fakepaper56]

Rebase and ping.

[reames]

There's missing context in the review description which is important. Please add something along the lines of the following.

The currently ratified calling convention in psABI does not support passing vector arguments or return values in registers. LLVM implements a non-standard calling convention for vector arguments and returns. psABI requires that such non-standard calling conventions be tagged w/ .variant_cc.

With that additional context, LGTM.

[fakepaper56]

Refine commit message. Almost add all reame's description into the the commit message.
Thank reame's suggestion.