IWYU: Fix handling of <memory> standard header

An old workaround for `std::allocator_traits<>::value_type` lints from
IWYU on `std::vector<>` usage breaks IWYU's handling of `<memory>`.
Convert the workaround to use the same approach we already use for a
workaround of `std::__decay_and_strip<>::::__type` lints.  Then update
the `<memory>` inclusions to follow the now-correct IWYU lints.

Tests/CMakeServerLib/testServerBuffering.cpp

@@ -1,6 +1,8 @@
 #include "cmConnection.h"
 #include "cmServerConnection.h"
+
 #include <iostream>
+#include <memory>
 #include <string>
 #include <vector>