cpp_transpiler should emit an array of all DSLX enum members

[mikex-oss]

What's hard to do? (limit 100 words)

The generated C++ library for DSLX is great! But it's unfortunate that C++ doesn't allow iterating over enum classes. However, since we generate the code, it'd be great to provide this in the generated C++ for the converted DSLX enum definitions. I think proto enums have something similar?

One use case: Run every case of this enum through some C++ test.

Current best alternative workaround (limit 100 words)

Manually reference each enum member, possibly missing some.

Your view of the "best case XLS enhancement" (limit 100 words)

We should generate an array like kAllFoos in addition to enum class Foo. @meheff pointed to some similar generated code for ops here:

xls/xls/ir/op.h

Lines 34 to 46 in 9a266ca

	// Enumerates the operator for nodes in the IR.
	enum class Op : int8_t {
	#define MAKE_ENUM(name, a, b, c) name,
	XLS_FOR_EACH_OP_TYPE(MAKE_ENUM)
	#undef MAKE_ENUM
	};
	// List of all the operators for nodes in the IR.
	constexpr auto kAllOps = std::to_array<Op>({
	#define MAKE_ENUM_REF(name, a, b, c) Op::name,
	XLS_FOR_EACH_OP_TYPE(MAKE_ENUM_REF)
	#undef MAKE_ENUM_REF
	});