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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Probably because no one bothered to do it and preferred magic builtins. The ext_vector support was based off of a language so it is going to be more canonical.

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