PR21565 Add an egregious hack to support broken libstdc++ headers that declare
a member named 'swap' and then expect unqualified lookup for the name 'swap' in
its exception specification to find anything else.

Without delay-parsed exception specifications, this was ill-formed (NDR) by
[basic.scope.class]p1, rule 2. With delay-parsed exception specifications, the
call to 'swap' unambiguously finds the function being declared, which then
fails because the arguments don't work for that function.