[libc++][test] Fix MSVC warning C4127 in array.cons/initialization.pass.cpp (#79793)

This fixes MSVC warning C4127: conditional expression is constant.

Testing `TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED` by itself doesn't
emit this warning, but the condition here is more complicated. I'm
expanding the macro and mechanically simplifying the resulting code.

(Yeah, this warning is often annoying, and I introduced
`TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED` to avoid this warning
elsewhere, so it's disappointing that it doesn't make the compiler happy
here. If this change is undesirable, I can replace it with
`ADDITIONAL_COMPILE_FLAGS(cl-style-warnings)`, but ideally I'd like to
avoid having to suppress it.)

---------

Co-authored-by: Louis Dionne <[email protected]>

Author: StephanTLavavej

libcxx/test/std/containers/sequences/array/array.cons/initialization.pass.cpp

@@ -28,10 +28,16 @@ struct test_initialization {
             // Before C++20, default initialization doesn't work inside constexpr for
             // trivially default constructible types. This only apply to non-empty arrays,
             // since empty arrays don't hold an element of type T.
-            if (TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED || !std::is_trivially_default_constructible<T>::value) {
-                std::array<T, 1> a1; (void)a1;
-                std::array<T, 2> a2; (void)a2;
-                std::array<T, 3> a3; (void)a3;
+#if TEST_STD_VER < 20
+            if (!(TEST_IS_CONSTANT_EVALUATED && std::is_trivially_default_constructible<T>::value))
+#endif
+            {
+              std::array<T, 1> a1;
+              (void)a1;
+              std::array<T, 2> a2;
+              (void)a2;
+              std::array<T, 3> a3;
+              (void)a3;
             }
 
             std::array<NoDefault, 0> nodefault; (void)nodefault;