[clang] [Clang][RFC] Intrododuce a builtin to determine the number of (PR #131515)

[via cfe-commits]

cor3ntin wrote:

Please look at the doc/tests and confirm this would be suitable to implement [`std::exec::tag_of_t`](https://eel.is/c++draft/exec#snd.concepts-5) / P2300

@ldionne @jwakely @StephanTLavavej @philnik777 @ericniebler 

https://github.com/llvm/llvm-project/pull/131515