[clang] Don't wrap immediate invocations in ConstantExprs within constexpr initializers (PR #89565)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I don't have a great idea on this one, but losing the diagnostics seems incorrect.  I think the whole initializer should probably be treated in 1 shot as a constant expr (so move the ConstantExpr to the top (above the `ImplicitCastExpr` in your example), but we DEFINITELY should be getting some of those diagnostics.

https://github.com/llvm/llvm-project/pull/89565