[llvm] [TableGen] Use guarded assert in constexpr functions (PR #194728)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

> Why can't it just be assert (which seems to work everywhere?) and then the function returning some dummy value?

You mean "assert(false)"?  Because older GCC will complain that there is a call to a non-constexpr function.  With newer compilers the original "llvm_unreachable" was accepted, but a GCC 7.4.0 buildbot failed.

https://github.com/llvm/llvm-project/pull/194728