[clang] [Clang] Add warning message for C++17 alias template CTAD (PR #133806)

[Erich Keane via cfe-commits]

erichkeane wrote:

Please dont' merge this until it is rebased on top of @Sirrade's patch and the SemaInit.cpp uses the new interface instead.

https://github.com/llvm/llvm-project/pull/133806