devel/creduce: add crutch for using libc++ 14 with clang++ 8

C-Reduce still depends on devel/llvm80, and this is no longer compatible
with libc++ 14 or higher, since libc++ now requires support for the
__builtin_is_constant_evaluated() builtin function (see
https://github.com/llvm/llvm-project/commit/1123100a16a321d70508e2508ebc5d57ce7163dc
).

As a crutch, add a small header which defines a constexpr
__builtin_is_constant_evaluated() function, and include that at the top
of every compiled file, using -include, whenever the base system clang
version is 14 or higher.

Approved by: swills (maintainer)
MFH: 2022Q1