[test] Add regression test for fn builders
[test] Add regression test for fn builders

benlangmuir/swift,apple/swift,glessard/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,glessard/swift,apple/swift,roambotics/swift,benlangmuir/swift,glessard/swift,apple/swift,benlangmuir/swift,glessard/swift,glessard/swift,roambotics/swift,roambotics/swift,apple/swift,benlangmuir/swift,apple/swift,apple/swift,glessard/swift,roambotics/swift,benlangmuir/swift