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/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;