[llvm] [LLVMABI] Create a skeleton for AArch64 ABI handling (PR #216222)

[David Truby via llvm-commits]

DavidTruby wrote:

Sorry if this isn't the right place for discussion on this but I just had a quick question/observation from the Flang side

> The MLIR piece of this was designed from the beginning with the idea of it being used by Flang also. Because CIR uses its own types, there will be some additional work to bring this up with Flang using the built-in types and any FIR-specific types, but we hope the effort required will be limited and that the CIR implementation will be a useful guide.

I'm not sure we necessarily want to always use the platform ABI/PCS for Flang functions that aren't marked BIND(C) (i.e. that aren't marked as callable from C); since we don't have to care about being compatible with other compilers in that case we currently just use a flang-specific PCS. So I guess we'd need a way of marking functions as wanting to use this ABI handling rather than always using it?  Or conversely, marking functions as _not_ wanting to use it although that would mean almost all flang functions being marked that way.

https://github.com/llvm/llvm-project/pull/216222