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.c

@@ -17,6 +17,7 @@ int globalStructSize = sizeof(globalStruct);
 short globalArray[3] = {5, 6, 7};
 joined_t globalUnion;
 int globalUnionSize = sizeof(globalUnion);
+option_t globalOption = optionG;
 
 int fortytwo() {
 	return 42;