[clang] [clang] Introduce __builtin_splatvector (PR #212758)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

> > > > Is there even a platform-generic type for scalable vectors yet?
> > > 
> > > 
> > > Sadly, no.
> > 
> > 
> > That seems like the thing we should solve first. After that I wouldn't be surprised if we didn't need this builtin at all.
> 
> No, it's the other way round. When I tried to introduce a platform-generic vector type, one of the comments was we should first improve builtin coverage of the sizeless vector types...

This is basically adding `VectorOfT(T)` though (assuming we end up designing a scalable vector type in a similar way to `ext_vector_type`). We don't need another builtin for that.

https://github.com/llvm/llvm-project/pull/212758