cgo: improve typedef/struct/enum support

Typedefs are now Go type aliases. And C.struct_ and C.union_ prefixed
records work correctly now, even when they're not in a typedef.

testdata/cgo/main.h

@@ -15,6 +15,11 @@ typedef struct collection {
 	unsigned char c;
 } collection_t;
 
+struct point {
+	int x;
+	int y;
+};
+
 // linked list
 typedef struct list_t {
 	int           n;