[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).

CC @AaronBallman regarding whether this needs an RFC

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