Expand isTypeExprTopDown

We should be using all subtypes of `FieldBase`. This allows us to find
more type expressions, and is also simpler to evaluate.

Author: owen-mc

go/ql/lib/semmle/go/Expr.qll

@@ -2155,20 +2155,12 @@ private predicate isTypeExprTopDown(Expr e) {
   or
   e = any(ArrayTypeExpr ae).getElement()
   or
-  e = any(FieldDecl f).getTypeExpr()
-  or
-  e = any(ParameterDecl pd).getTypeExpr()
+  e = any(FieldBase fb).getTypeExpr()
   or
   e = any(TypeParamDecl tpd).getTypeConstraintExpr()
   or
   e = any(TypeParamDecl tpd).getNameExpr(_)
   or
-  e = any(ReceiverDecl rd).getTypeExpr()
-  or
-  e = any(ResultVariableDecl rvd).getTypeExpr()
-  or
-  e = any(MethodSpec md).getTypeExpr()
-  or
   e = any(MapTypeExpr mt).getKeyTypeExpr()
   or
   e = any(MapTypeExpr mt).getValueTypeExpr()