C front-end: fix initialisation of large arrays

In 375e9a8 the construction of zero initialisers was switched to
array_of expressions for arrays with more than 1000 elements. This is
beneficial to avoid running out of memory, but didn't take into account
arrays with designated initialisers, which this patch fixes.

Author: tautschnig

regression/ansi-c/array_initialization6/main.c

@@ -0,0 +1,7 @@
+// an array that is larger than MAX_FLATTENED_ARRAY_SIZE
+static const unsigned char data[256 * 4] = {0x0, 0x0};
+
+int main()
+{
+  return data[0];
+}

regression/ansi-c/array_initialization6/test.desc

@@ -0,0 +1,7 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring

src/ansi-c/c_typecheck_initializer.cpp

@@ -397,6 +397,25 @@ exprt::operandst::const_iterator c_typecheck_baset::do_designated_initializer(
     if(full_type.id()==ID_array ||
        full_type.id()==ID_vector)
     {
+      // zero_initializer may have built an array_of expression for a large
+      // array; as we have a designated initializer we need to have an array of
+      // individual objects
+      if(dest->id() == ID_array_of)
+      {
+        const array_typet array_type = to_array_type(dest->type());
+        const auto array_size = numeric_cast<mp_integer>(array_type.size());
+        if(!array_size.has_value())
+        {
+          error().source_location = value.source_location();
+          error() << "cannot zero-initialize array with subtype '"
+                  << to_string(full_type.subtype()) << "'" << eom;
+          throw 0;
+        }
+        const exprt zero = to_array_of_expr(*dest).what();
+        *dest = array_exprt{{}, array_type};
+        dest->operands().resize(numeric_cast_v<std::size_t>(*array_size), zero);
+      }
+
       if(index>=dest->operands().size())
       {
         if(full_type.id()==ID_array &&