[libc++] Add the __is_replaceable type trait (#132408)

That type trait represents whether move-assigning an object is
equivalent to destroying it and then move-constructing a new one from
the same argument. This will be useful in a few places where we may want
to destroy + construct instead of doing an assignment, in particular
when implementing some container operations in terms of relocation.

This is effectively adding a library emulation of P2786R12's
is_replaceable trait, similarly to what we do for trivial relocation.
Eventually, we can replace this library emulation by the real
compiler-backed trait.

This is building towards #129328.

libcxx/include/module.modulemap.in

@@ -272,6 +272,10 @@ module std_core [system] {
       header "__type_traits/is_referenceable.h"
       export std_core.type_traits.integral_constant
     }
+    module is_replaceable {
+      header "__type_traits/is_replaceable.h"
+      export std_core.type_traits.integral_constant
+    }
     module is_same {
       header "__type_traits/is_same.h"
       export std_core.type_traits.integral_constant