Propagate Pattern Arena When Allocating PackExpansionTypes
Propagate Pattern Arena When Allocating PackExpansionTypes

This may be the cause of the instability we've been seeing on the bots
here. If a type variable is used as the pattern type across allocation
arenas, we may incorrectly reuse a cache entry and it could be
substituted a second time for a bogus type.

rdar://87413287

rudkx/swift,gregomni/swift,ahoppen/swift,glessard/swift,benlangmuir/swift,JGiola/swift,rudkx/swift,benlangmuir/swift,roambotics/swift,atrick/swift,atrick/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,glessard/swift,atrick/swift,rudkx/swift,glessard/swift,gregomni/swift,ahoppen/swift,gregomni/swift,rudkx/swift,apple/swift,gregomni/swift,apple/swift,rudkx/swift,gregomni/swift,ahoppen/swift,glessard/swift,apple/swift,apple/swift,JGiola/swift,apple/swift,atrick/swift,roambotics/swift,ahoppen/swift,glessard/swift,atrick/swift,glessard/swift,ahoppen/swift,benlangmuir/swift,gregomni/swift,atrick/swift,benlangmuir/swift,JGiola/swift,ahoppen/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,roambotics/swift,roambotics/swift,apple/swift,rudkx/swift