asn1: note IMPLICIT CHOICE promoted to EXPLICIT

Record when a CHOICE field is promoted from IMPLICIT to EXPLICIT and convey
this in the ASN.1 compiler's JSON output, so that other tools (e.g. which have
a representation isomorphic to the original ASN.1) may use it.

lib/asn1/asn1parse.y

@@ -1432,8 +1432,10 @@ TaggedType	: Tag tagenv Type
 			     * IMPLICIT tags of CHOICE types are EXPLICIT
 			     * instead.
 			     */
-			    if (t->type == TChoice)
+			    if (t->type == TChoice) {
+				$$->implicit_choice = 1;
 				$$->tag.tagenv = TE_EXPLICIT;
+			    }
 			    if($3->type == TTag && $2 == TE_IMPLICIT) {
 				$$->subtype = $3->subtype;
 				free($3);

lib/asn1/gen.c

@@ -1529,6 +1529,9 @@ define_type(int level, const char *name, const char *basename, Type *pt, Type *t
         fprintf(jsonfile, "\"ttype\":\"TeletexString\",\"ctype\":\"heim_general_string\"");
 	break;
     case TTag:
+        if (t->implicit_choice) {
+            fprintf(jsonfile, "\"desired_tagenv\":\"IMPLICIT\",");
+        }
         fprintf(jsonfile, "\"tagclass\":\"%s\",\"tagvalue\":%d,\"tagenv\":\"%s\",\n",
                 tagclassnames[t->tag.tagclass], t->tag.tagvalue,
                 t->tag.tagenv == TE_EXPLICIT ? "EXPLICIT" : "IMPLICIT");

lib/asn1/symbol.h

@@ -200,6 +200,7 @@ struct type {
     struct range *range;
     struct constraint_spec *constraint;
     unsigned long id;
+    unsigned int implicit_choice:1;
 };
 
 typedef struct type Type;