debuginfo: Don't mark struct fields as artificial.

LLDB doesn't allow for reading 'artifical' fields (fields that are generated by the compiler). So do not mark, slice fields, enum discriminants, and GcBox value fields as artificial.

src/etc/gdb_rust_pretty_printing.py

@@ -196,7 +196,7 @@ def classify_struct(type):
   if field_count == 0:
     return STRUCT_KIND_REGULAR_STRUCT
 
-  if fields[0].artificial:
+  if fields[0].name == "RUST$ENUM$DISR":
     if field_count == 1:
       return STRUCT_KIND_CSTYLE_VARIANT
     elif fields[1].name == None: