[clang] [clang] Turn -Wenum-constexpr-conversion into a hard error (PR #102364)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > How soon after 1.86 landing do you plan to submit this?
> 
> I was thinking as soon as it's released, I don't see a reason for waiting any longer. The sooner we merge the sooner we can collect feedback and re-adjust if needed. But of course it's up to the Clang owners to decide :)

I think that's a reasonable plan -- if we find it's extremely disruptive, we can cross that bridge when we come to it.

https://github.com/llvm/llvm-project/pull/102364