CXX-2625 Address sign conversion warnings for literal argument to make_unique<T[]>()

[eramongodb]

Address the following warning:

warning: implicit conversion changes signedness: 'int' to 'std::size_t' (aka 'unsigned long') [-Wsign-conversion]
    return Impl::make(std::true_type{}, std::forward<Args>(args)...);
           ~~~~                         ^~~~~~~~~~~~~~~~~~~~~~~~
note: in instantiation of function template specialization 'bsoncxx::stdx::make_unique<int[], int, bsoncxx::stdx::detail::make_unique_impl<int[]>, std::unique_ptr<int[]>>' requested here
    auto ptr = bsoncxx::stdx::make_unique<int[]>(3);
                              ^
1 warning generated.

given the following code:

auto ptr = bsoncxx::stdx::make_unique<int[]>(3);

for conformance with specification that states there should be a separate overload for the unbounded array case with an explicit single std::size_t parameter which integral literals that are representable by std::size_t should be able to initialize without triggering a warning.

Proposed changes make use of the std::enable_if_t<cond, decltype(expr, void())>* = nullptr pattern to define multiple SFINAE templates without conflicts using both a boolean condition cond and an expression validity check expr. Note: uniqueness of both the condition and the expression is encoded in the specialization of std::enable_if as a non-type template parameter, avoiding the possibility of "template parameter redefines default argument" errors that is often encountered when using the typename = ... pattern for multiple overloads.

The std::enable_if<...>* = nullptr pattern can be changed to std::enable_if<...> = 0 if preferable. (Makes no singificant difference; just a style choice.)

Examples of warnings and errors on expression validity failure with proposed changes:

auto ptr = bsoncxx::stdx::make_unique<int[]>(3);  // No warnings.
auto ptr = bsoncxx::stdx::make_unique<int[]>(3u); // No warnings.

warning: implicit conversion changes signedness: 'int' to 'std::size_t' (aka 'unsigned long') [-Wsign-conversion]
    return Impl::make(std::true_type{}, std::forward<Args>(args)...);
           ~~~~                         ^~~~~~~~~~~~~~~~~~~~~~~~
note: in instantiation of function template specialization 'bsoncxx::stdx::make_unique<int[], int, bsoncxx::stdx::detail::make_unique_impl<int[]>, std::unique_ptr<int[]>>' requested here
    auto ptr = bsoncxx::stdx::make_unique<int[]>(-1);
                              ^

error: no matching function for call to 'make_unique'
    auto ptr = bsoncxx::stdx::make_unique<int[3]>(3);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: candidate template ignored: substitution failure [with T = int[3], Args = <int>, Impl = bsoncxx::stdx::detail::make_unique_impl<int[3]>]: no member named 'make' in 'bsoncxx::stdx::detail::make_unique_impl<int[3]>'
std::unique_ptr<T> make_unique(Args&&... args) {
                   ^

[vector-of-bool]

Good find. I hadn't realized this.