Add asserts in init methods

Author: kimdv

Sources/SwiftSyntaxBuilder/Buildables.swift.gyb

@@ -136,6 +136,20 @@ public struct ${node.syntax_kind}: ${node.base_kind}Buildable {
 %       child_token = SYNTAX_TOKEN_MAP.get(child.syntax_kind)
 %       param_type = syntax_buildable_child_type(child.type_name, child.syntax_kind, child.is_token())
     self.${child.swift_name} = ${child.swift_name}${create_method_dot}create${param_type}()
+%       assert_choices = []
+%       if child.text_choices:
+%         assert_choices = map(lambda x : '"' + x + '"', child.text_choices)
+%       elif child.token_choices:
+%         assert_choices = map(lambda x : '"' + SYNTAX_TOKEN_MAP.get(x.name + 'Token').text + '"', child.token_choices)
+%         # some cases, like IdentifierToken, will add an empty string.
+%         # So if there is any empty string we just make it empty
+%         assert_choices = [] if '""' in assert_choices else assert_choices
+%       end
+%       if assert_choices:
+%         optional_condition = child.swift_name + ' == nil || ' if child.is_optional else ""
+%         unwrap = "!" if child.is_optional else ""
+    assert(${optional_condition}[${', '.join(assert_choices)}].contains(self.${child.swift_name}${unwrap}.text))
+%       end
 %     end
   }