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

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

> These functions take a scoped enum as an argument, and have a switch that covers all cases

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

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