libctf: get the offsets of fields of unnamed structs/unions right

We were failing to add the offsets of the containing struct/union
in this case, leading to all offsets being relative to the unnamed
struct/union itself.

libctf/
	PR libctf/30264
	* ctf-types.c (ctf_member_info): Add the offset of the unnamed
	member of the current struct as necessary.
	* testsuite/libctf-lookup/unnamed-field-info*: New test.

libctf/testsuite/libctf-lookup/unnamed-field-info-ctf.c

@@ -0,0 +1,36 @@
+struct A
+{
+  int a;
+  char *b;
+  struct
+  {
+    struct
+    {
+      char *one;
+      int two;
+    };
+    union
+    {
+      char *three;
+    };
+  };
+  struct
+  {
+    int four;
+  };
+  union
+  {
+    struct
+    {
+      double x;
+      long y;
+    };
+    struct
+    {
+      struct { char *foo; } z;
+      float aleph;
+    };
+  };
+};
+
+struct A used;