[llvm] [clang] [RISCV] CodeGen of RVE and ilp32e/lp64e ABIs (PR #76777)

[Alex Bradbury via cfe-commits]

asb wrote:

The conclusion from the previous review was this was OK to merge. I think I held it up by not responding to a ping (apologies). I've had another scan through and don't see a problem with merging this and considering it experimental once Craig's review comments are addressed.

For the psABI, there's a gap here I think as GCC and LLVM need to deviate from what is currently written. I think https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/257 tried to partially address this. I wonder if we need a tracking issue in that repo to try to get this properly documented via that PR or otherwise (what do you think @jrtc27 @kito-cheng?).

https://github.com/llvm/llvm-project/pull/76777