[clang] [clang] add `-fimplicit-constexpr` flag (PR #136436)

[via cfe-commits]

Sirraide wrote:

> IMO this kind of flag requires an RFC. This is a non-conforming extension and I don't see any rationale for why we'd want to add it.

I feel like GCC compatibility might be an argument? Imo it depends a bit on how difficult it is to implement the flag, but an RFC to discuss the exact behaviour of it might be warranted (e.g. do we require `inline` or not—unless we want to just do the *exact* same thing as GCC).

https://github.com/llvm/llvm-project/pull/136436