[CS] Add a fallback for overload locators that failed to simplify
[CS] Add a fallback for overload locators that failed to simplify

If an overload locator couldn't be simplified to its anchor, fallback to
the anchor of the locator instead. Also add an assert so that we can
track down whether this is actually valid or not.

Resolves rdar://89097800.

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