[clang] [Clang] Implement P2747 constexpr placement new (PR #104586)

[via cfe-commits]

cor3ntin wrote:

> Thank you for working on this! Given that it's an extension in C++20 mode, we're missing diagnostics and test coverage for issuing extension/precompat diagnostics.

No longer an extension after our last chat - i just forgot to update the description

https://github.com/llvm/llvm-project/pull/104586