[libc++][test] Avoid MSVC constexpr bug

[CaseyCarter]

Summary

C++ constexpr allows a non-constant-expresssion lvalue to be used in a constant expression if it's not subject to lvalue-to-rvalue conversion. Subtly, this means you can make a constant-expression copy of a non-constant-expression object of empty type since the copy constructor doesn't perform lvalue-to-rvalue conversion. MSVC has had bugs with this usage forever, which will hopefully finally be mashed implementing C++23's relaxation on the use of pointers and references in constant expressions.

There's no need for this particular test to use this particular constexpr feature, we can simply make the predicates constant expressions.

[philnik]

I had no idea this is possible. LGTM.

libcxx/test/std/algorithms/ranges_robust_against_nonbool_predicates.pass.cpp

 #include <array>
 #include <concepts>
 #include <initializer_list>
 #include <iterator>
 #include <ranges>
 
 #include "boolean_testable.h"
 
-auto unary_pred = [](int i) { return BooleanTestable(i > 0); };
+constexpr auto unary_pred = [](int i) { return BooleanTestable(i > 0); };
 static_assert(!std::same_as<decltype(unary_pred(1)), bool>);
 static_assert(std::convertible_to<decltype(unary_pred(1)), bool>);
 
-auto binary_pred = [](int i, int j) { return BooleanTestable(i < j); };
+constexpr auto binary_pred = [](int i, int j) { return BooleanTestable(i < j); };
 static_assert(!std::same_as<decltype(binary_pred(1, 2)), bool>);
 static_assert(std::convertible_to<decltype(binary_pred(1, 2)), bool>);
 
 // Invokes both the (iterator, sentinel, ...) and the (range, ...) overloads of the given niebloid.
 
 // (in, ...)
 template <class Func, std::ranges::range Input, class... Args>
 constexpr void test(Func&& func, Input& in, Args&&... args) {