fix anonymous struct member initialization

The C front-end needs to ignore the unnamed bit-field during struct
initialization; issue #3709.

regression/cbmc/Struct_Initialization1/main.c

@@ -3,7 +3,9 @@
 struct tag1 {
   int f;
   int : 32; // unnamed bit-field, ignored during initialization
-  int g;
+  union {
+    int g;
+  }; // anonymous, but not ignored
   int *p;
   int a[2];
 } x;

regression/cbmc/Struct_Initialization1/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 
 ^EXIT=0$

src/ansi-c/c_typecheck_initializer.cpp

@@ -665,10 +665,16 @@ void c_typecheck_baset::increment_designator(designatort &designator)
       assert(components.size()==entry.size);
 
       // we skip over any padding or code
-      while(entry.index<entry.size &&
+      // we also skip over anonymous members
+      while(entry.index < entry.size &&
             (components[entry.index].get_is_padding() ||
-             components[entry.index].type().id()==ID_code))
+             (components[entry.index].get_anonymous() &&
+              components[entry.index].type().id() != ID_struct_tag &&
+              components[entry.index].type().id() != ID_union_tag) ||
+             components[entry.index].type().id() == ID_code))
+      {
         entry.index++;
+      }
 
       if(entry.index<entry.size)
         entry.subtype=components[entry.index].type();