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

[Paul Osmialowski via cfe-commits]

pawosm-arm wrote:

@jhuber6 and @efriedma-quic do you know why this kind of (splat) initialization in C++ has only been introduced to the `ext_vector_type` types? Why can't it also be available generally for fixed-length and scalable vectors?

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