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.

Source/cmLocalUnixMakefileGenerator3.cxx

@@ -5,7 +5,7 @@
 #include "cmsys/FStream.hxx"
 #include "cmsys/Terminal.h"
 #include <algorithm>
-#include <memory> // IWYU pragma: keep
+#include <memory>
 #include <sstream>
 #include <stdio.h>
 #include <utility>