cgo: add support for enum types

Enum types are implemented as named types (with possible accompanying
typedefs as type aliases). The constants inside the enums are treated as
Go constants like in the Go toolchain.

testdata/cgo/main.go

@@ -75,6 +75,24 @@ func main() {
 		println("n in chain:", list.n)
 		list = (*C.list_t)(list.next)
 	}
+
+	// named enum
+	var _ C.enum_option = C.optionA
+	var _ C.option_t = C.optionA
+	println("option:", C.globalOption)
+	println("option A:", C.optionA)
+	println("option B:", C.optionB)
+	println("option C:", C.optionC)
+	println("option D:", C.optionD)
+	println("option E:", C.optionE)
+	println("option F:", C.optionF)
+	println("option G:", C.optionG)
+
+	// anonymous enum
+	var _ C.option2_t = C.option2A
+	var _ C.option3_t = C.option3A
+	println("option 2A:", C.option2A)
+	println("option 3A:", C.option3A)
 }
 
 func printUnion(union C.joined_t) C.joined_t {