swift vdl codegen: Allow overflow when computing struct hashValue

DJB hash computation of Swift code generated from VDL must allow
overflow of the * and + operators, otherwise an exception is thrown.
The hash value is still valid even if overflow occurs; it just wraps
in hash-space.

Change-Id: I6832d7105a8f677dea593828c2de5e4083e7f62f
diff --git a/lib/vdl/codegen/swift/tmpl_struct.go b/lib/vdl/codegen/swift/tmpl_struct.go
index 130fc19..89c2f14 100644
--- a/lib/vdl/codegen/swift/tmpl_struct.go
+++ b/lib/vdl/codegen/swift/tmpl_struct.go
@@ -40,7 +40,7 @@
     {{ .AccessModifier }} var hashValue: Int {
       var result = 1
       let prime = 31
-      {{ range $field := .Fields }}result = prime * result + Int({{$field.HashcodeComputation}})
+      {{ range $field := .Fields }}result = prime &* result &+ Int({{$field.HashcodeComputation}})
       {{ end }}
       return result
     }