A wrapper type to ensure that the parser handles outer attributes correctly.
When we parse outer attributes, we need to ensure that we capture tokens
for the attribute target. This allows us to perform cfg-expansion on
a token stream before we invoke a derive proc-macro.
Converts a flattened iterator of tokens (including open and close delimiter tokens) into an
AttrTokenStream, creating an AttrTokenTree::Delimited for each matching pair of open and
close delims.