AST: Avoid invoking TypeAliasDecl::getUnderlyingType() from isInterestingTypealias()
AST: Avoid invoking TypeAliasDecl::getUnderlyingType() from isInterestingTypealias()

Diagnostics are emitted from weird places sometimes, and as a result the emission
is fragile for two reasons:

- Kicking off requests can trigger cycles
- Doing anything that can re-entrantly diagnose something else might crash

In this case, the problem is getUnderlyingType(), which might kick off type
resolution on the typealias. Since we already have a Type, just check that
instead of getting a new one.

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