Allow ADL for swapping Optional values (#4569)

Some types support being swapped but may not have declared their
std::swap overloads when Poco/Optional.h is first included. This is the
case for instance with

    #include <Poco/Optional.h>
    #include <array>

    using Problematic = Poco::Optional<std::array<int, 42> >;

With an unqualified call to swap, preceded by using std::swap, we allow
argument-dependent lookup to find suitable implementations of swap.

Foundation/include/Poco/Optional.h

@@ -143,8 +143,9 @@ class Optional
 
 	void swap(Optional& other) noexcept
 	{
-		std::swap(_value, other._value);
-		std::swap(_isSpecified, other._isSpecified);
+		using std::swap;
+		swap(_value, other._value);
+		swap(_isSpecified, other._isSpecified);
 	}
 
 	const C& value() const