[llvm] [TableGen] Emit constexpr versions of some directive/clause functions (PR #194633)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> Why do we need llvm_unreachable/assert if all cases are covered?

That's the convention used throughout these functions and I didn't want to start a discussion about whether we want it or not since it didn't affect what I wanted to do.

https://github.com/llvm/llvm-project/pull/194633