C++ interop: add a simple unit test for "using enum"

[neundorf]

This unit test uses "using UEnum = Enum;" and imports that into Carbon.

This is my very first try at contributing something to carbon, I'm looking forward to your feedback.
The test is very basic.
What other cases should it test ?
What other comments do you have ?

[neundorf]

The test currently simply tests one using enum on global scope.
Should I add an enum inside a class ? An enum class inside a class ? Other variations ?

[zygoloid]

If this is for the "using enum support" item in the toolchain tasks document, that's talking about the C++20 using enum feature:

enum class A { x, y };
// Cannot name `A::x` as simply `x` here.
using enum A;
// Can now name `A::x` as simply `x`.
A a = x;

The using B = A; syntax is an alternative syntax for typedef declarations, and we have tests for that in toolchain/check/testdata/interop/cpp/typedef.carbon already.