constexpr: add support for tagging address constants
Address constants [6.6(9)] constitute one of the types of constant
expressions allowed in initializers [6.6(7)] for static storage
duration objects [6.7.8(4)].
Introduce a new flag for tagging expressions which qualify as
being an address constant.
Make sure not to carry over the address constant attribute from
subexpressions for operators that never yield address constants,
i.e. most arithmetic ones, logical ones etc.
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>