C front-end: maintain alignment attribute for struct members

Ensure that struct/union typed members of structs with alignment
attributes keep these annotations during type checking, which were
already correctly preserved for typedef-typed members.

Author: tautschnig

regression/ansi-c/Struct_Padding7/main.c

@@ -0,0 +1,33 @@
+#define CONCAT(a, b) a##b
+#define CONCAT2(a, b) CONCAT(a, b)
+
+#define STATIC_ASSERT(condition)                                               \
+  int CONCAT2(some_array, __LINE__)[(condition) ? 1 : -1]
+
+typedef struct S1
+{
+  int x;
+} S1;
+
+#ifdef __GNUC__
+struct S2
+{
+  struct S1 __attribute__((__aligned__(((1L) << 12)))) s1;
+};
+
+struct foo
+{
+  char a;
+  int x[2] __attribute__((packed));
+};
+#endif
+
+int main()
+{
+#ifdef __GNUC__
+  STATIC_ASSERT(sizeof(struct S1) == sizeof(int));
+  STATIC_ASSERT(sizeof(struct S2) == (1L << 12));
+  STATIC_ASSERT(sizeof(struct foo) == sizeof(char) + 2 * sizeof(int));
+#endif
+  return 0;
+}

regression/ansi-c/Struct_Padding7/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/c_typecheck_type.cpp

@@ -761,6 +761,9 @@ void c_typecheck_baset::typecheck_compound_type(struct_union_typet &type)
     original_qualifiers.is_transparent_union;
   remove_qualifiers.write(type);
 
+  bool is_packed = type.get_bool(ID_C_packed);
+  irept alignment = type.find(ID_C_alignment);
+
   if(type.find(ID_tag).is_nil())
   {
     // Anonymous? Must come with body.
@@ -867,6 +870,11 @@ void c_typecheck_baset::typecheck_compound_type(struct_union_typet &type)
   type.swap(tag_type);
 
   original_qualifiers.write(type);
+
+  if(is_packed)
+    type.set(ID_C_packed, true);
+  if(alignment.is_not_nil())
+    type.set(ID_C_alignment, alignment);
 }
 
 void c_typecheck_baset::typecheck_compound_body(