cgo: support anonymous enums included in multiple Go files

Anonymous enums (often used in typedefs) triggered a problem that was
already solved for structs but wasn't yet solved for enums. So this
patch generalizes the code to work for both structs and enums, and adds
testing for both.

testdata/cgo/test.h

@@ -0,0 +1,11 @@
+#pragma once
+
+typedef struct {
+    int a;
+    int b;
+} teststruct;
+
+typedef enum {
+    v0,
+    v1
+} testenum;