[libc++][test] Do not test Clang bug in is_constructible.pass.cpp (#105964)

A comment in `is_constructible.pass.cpp` suggests that Clang is
non-conforming in accepting construction of `const int&` from
`ExplicitTo<int&&>`.

This PR changes the test to expect the standard-conforming behavior,
which makes the test pass on MSVC.

Author: cpplearner

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_constructible.pass.cpp

@@ -228,8 +228,8 @@ int main(int, char**)
     // But the rvalue to lvalue reference binding isn't allowed according to
     // [over.match.ref] despite Clang accepting it.
     test_is_constructible<int&, ExplicitTo<int&>>();
-#ifndef TEST_COMPILER_GCC
-    test_is_constructible<const int&, ExplicitTo<int&&>>();
+#ifndef TEST_COMPILER_CLANG
+    test_is_not_constructible<const int&, ExplicitTo<int&&>>();
 #endif
 
     static_assert(std::is_constructible<int&&, ExplicitTo<int&&>>::value, "");