[clang] [clang][Sema] Allow splat initialization of a sizeless vector in a C++ code (PR #205432)

[Paul Osmialowski via cfe-commits]

pawosm-arm wrote:

> From the commit message I'd assumed this was allowing something already supported generally for fixed-length vectors. However, from testing locally, I see we don't allow this for standard NEON types (e.g. https://godbolt.org/z/5bTaq87ve). I guess it's only `ext_vector_type` that allow these implicit conversions.
> 
> With your patch applied these conversions work (and result in splats), but only for SVE types: https://godbolt.org/z/8b59KKGcG
> 
> Is this intentional? I'd assumed this was about parity between fixed/scalable types (and/or NEON/SVE types).

Yes, and the commit message says it all (about the scope). This is the part of the effort behind this RFC: https://discourse.llvm.org/t/rfc-clang-libc-extend-the-ext-vector-type-attribute-to-support-the-scalable-vector-sizes/90043

https://github.com/llvm/llvm-project/pull/205432