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

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

Can you explain how this links to that RFC? As this PR is changing the behaviour for builtin scalable vector types, not the `ext_vector_type`. I think for that RFC you'd still limit changes to `ext_vector_type`.

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