[clang] [clang][Sema] support block pointers as non-type template parameters (PR #191694)

[via cfe-commits]

Serosh-commits wrote:

> Yeah we don't need ExprConstant support for starters.
> 
> To be honest, we really don't need anything, and I am assuming you are doing this for the learning experience.
> 
> The Apple folks would be really the only ones with any use for this, but they never bothered adding this support, so I don't think user-base-wise the motivation is there.
> 
> If adding support for passing these pointers as template parameters turns out to be simple and low effort to maintain, I would have no objections to merging this.
> 
> The same applies to ExprConstant. If the patch is too invasive and complex, the maintainer might take issue. Never mind that we have two constant expression evaluators, there is one that is actually used by the users, and another one where all new development happens.
> 
> It doesn't look promising if you add support only to the old one, but then you are not getting any users if you support only the new one.

@mizvekov Makes sense. i will not make a pr for the ExprConstant evaluation support entirely so it doesn't add any complexity to the new/old evaluators
I'll keep the PR limited to as it is now  Let me know if that looks good to merge once I push the update

https://github.com/llvm/llvm-project/pull/191694