[Macros] Eliminate ordering dependency that suppressed conformingTo protocols
When a macro that has both 'member' and 'extension' roles is on a type,
and both list conforming protocols, the order in which those roles were
evaluated in the compiler could change the set of protocols passed to
the macro expansion function (via `conformingTo:`). Specifically, if
the extension macro was expanded first, the member macro would see the
extension providing the conformance to one of its protocols, and not
pass down that protocol to the member macro's `conformingTo:`.
Ensure that we account for already-expanded extension macros that
define conformances.
Fixes rdar://137080876.