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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Implicit conversions with vectors has been a mess historically.  I'd be happier with a builtin, I think.

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