[clang] [Clang] Amend SME attributes with support for ZT0. (PR #77941)

[Sander de Smalen via cfe-commits]

sdesmalen-arm wrote:

> I won't approve because of the growth in FunctionProtoType — someone more qualified than me should sign off on that.

@AaronBallman or @erichkeane, would you be happy to sign off on this patch?

I think previously you raised concerns about the size of `ExtProtoInfo` when adding more bits to it. (This was discussed on https://reviews.llvm.org/D127762, but unfortunately this link currently results in a 404)

It's worth pointing out that extending `ExtProtoInfo` with some more bits doesn't change the actual size. It is 80 bytes at the moment and increasing it with 2 more bits doesn't change the total size of the struct. The size of `FunctionTypeExtraBitfields` will increase though, but I'm not sure if this is a problem because the comment above it says that these bits are uncommon to start with. I could shave off another two bits off `NumExceptionType` to avoid that, as the comment suggests 8 bits should be sufficient.

This also make me wonder if we should be reserving some extra bits for future state that may need adding?

https://github.com/llvm/llvm-project/pull/77941