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

[Nikolas Klauser via cfe-commits]

philnik777 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.

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