Add support of extension group inside CHOICE type

In LTE-RRC 36.331 15.3.0, there is :

    RLC-Config-NB-r13 ::= CHOICE {
        am    SEQUENCE {
            ul-AM-RLC-r13                UL-AM-RLC-NB-r13,
            dl-AM-RLC-r13                DL-AM-RLC-NB-r13
        },
        ...,
        [[  um-Bi-Directional-r15        NULL,
            um-Uni-Directional-UL-r15    NULL,
            um-Uni-Directional-DL-r15    NULL
        ]]
    }

that asn1c can not handle `[[` ... `]]` inside CHOICE type.

1. Modify asn1p_y.y and asn1p_y.c to mimic the approach for handling extension
   group inside SEQUENCE type.

2. Modify asn1c_C.c to use 'ext1', 'ext2' ... as extension groups names.

libasn1compiler/asn1c_C.c

@@ -971,6 +971,7 @@ asn1c_lang_C_type_CHOICE(arg_t *arg) {
 	asn1p_expr_t *expr = arg->expr;
 	asn1p_expr_t *v;
 	int saved_target = arg->target->target;
+	int ext_num = 1;
 
 	DEPENDENCIES;
 
@@ -990,6 +991,14 @@ asn1c_lang_C_type_CHOICE(arg_t *arg) {
 				skipComma = 1;
 				continue;
 			}
+
+			if((v->expr_type == ASN_CONSTR_SEQUENCE) &&
+				(v->marker.flags & EM_OPTIONAL) &&
+				(v->Identifier == NULL)) {
+				char ext_name[20];
+				sprintf(ext_name, "ext%d", ext_num++);
+				v->Identifier = strdup(ext_name);
+			}
 			OUT("%s", c_presence_name(arg, v));
 		}
 		OUT("\n");

libasn1parser/asn1p_y.y

@@ -1075,6 +1075,13 @@ AlternativeTypeLists:
 		$$ = $1;
 		asn1p_expr_add($$, $3);
 	}
+	| AlternativeTypeLists ',' TOK_VBracketLeft AlternativeTypeLists TOK_VBracketRight {
+		$$ = $1;
+		$4->meta_type = AMT_TYPE;
+		$4->expr_type = ASN_CONSTR_SEQUENCE;
+		$4->marker.flags |= EM_OPTIONAL;
+		asn1p_expr_add($$, $4);
+	}
 	;
 
 AlternativeType: