libdrgn: get rid of arrays embedded in drgn_type

For C++ support, we need to add an array of template parameters to
struct drgn_type. struct drgn_type already has arrays for members,
enumerators, and parameters embedded at the end of the structure,
because no type needs more than one of those. However, struct, union,
and class types may need members and template parameters. We could add a
separate array of templates, but then it gets confusing having two
methods of storing arrays in struct drgn_type. Let's make these arrays
separate instead of embedding them.

Author: jgkamat

libdrgn/drgn.h.in

@@ -446,15 +446,12 @@ struct drgn_type {
 			size_t num_members;
 			struct drgn_type *type;
 		};
+		union {
+			struct drgn_type_member *members;
+			struct drgn_type_enumerator *enumerators;
+			struct drgn_type_parameter *parameters;
+		};
 	} _private;
-	/*
-	 * An array of struct drgn_type_member, struct drgn_type_enumerator, or
-	 * struct drgn_type_parameter may follow. We can't use flexible array
-	 * members for these because they are not allowed in a union or nested
-	 * structure; we can't use GCC's zero length array extension because
-	 * that triggers false positives in Clang's AddressSanitizer. Instead,
-	 * these are accessed internally with drgn_type_payload().
-	 */
 };
 
 /**
@@ -606,11 +603,6 @@ static inline const char *drgn_type_tag(struct drgn_type *type)
 	return type->_private.tag;
 }
 
-static inline void *drgn_type_payload(struct drgn_type *type)
-{
-	return (char *)type + sizeof(*type);
-}
-
 /**
  * Get whether a kind of type has members. This is true for structure, union,
  * and class types.
@@ -633,7 +625,7 @@ static inline bool drgn_type_has_members(struct drgn_type *type)
 static inline struct drgn_type_member *drgn_type_members(struct drgn_type *type)
 {
 	assert(drgn_type_has_members(type));
-	return drgn_type_payload(type);
+	return type->_private.members;
 }
 /**
  * Get the number of members of a type. @ref drgn_type_has_members() must be
@@ -710,7 +702,7 @@ static inline struct drgn_type_enumerator *
 drgn_type_enumerators(struct drgn_type *type)
 {
 	assert(drgn_type_has_enumerators(type));
-	return drgn_type_payload(type);
+	return type->_private.enumerators;
 }
 /**
  * Get the number of enumerators of a type. @ref drgn_type_has_enumerators()
@@ -761,7 +753,7 @@ static inline bool drgn_type_has_parameters(struct drgn_type *type)
 static inline struct drgn_type_parameter *drgn_type_parameters(struct drgn_type *type)
 {
 	assert(drgn_type_has_parameters(type));
-	return drgn_type_payload(type);
+	return type->_private.parameters;
 }
 /**
  * Get the number of parameters of a type. @ref drgn_type_has_parameters() must