[llvm] [TableGen] Emit constexpr versions of some directive/clause functions (PR #194406)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> Assert in constexpr functions has been allowed since C++14, I think; it should work fine in compilers we support.

Indeed.

Replacing this PR with https://github.com/llvm/llvm-project/pull/194633.

https://github.com/llvm/llvm-project/pull/194406