GlobalFunctions: Deprecate swap()

GlobalFunctions: Deprecate swap()

This unprefixed function, added in r7198 (2b3f4c749d3d), has been
unused since r12411 (f2a59db33f33).

It is short and simple enough that on the rare occasions it is needed,
it can be inlined or copied into the calling class as a private method.

Alternatively, the idiom `list( $a, $b ) = array( $b, $a )` can be used.

Change-Id: Ieb4602597a54eb21a5de177fee6dafa7ac71ce1d