Include return param names in modified namespaced compilation

[ericglau]

When making a modified copy of solc input to extract namespaced storage layout, we converted function return variables to unnamed bools e.g. return (bool). This allowed compilation to pass even if the NatSpec was documented with the return variable name as @return x

However, if the NatSpec of the return variable was documented by the user as in the below example using @param x instead of @return x, this fails the modified compilation even though the original format was allowed by Solidity:

/*
 * @param x my param
 */
function foo() pure returns (uint x) {
}

We should keep the return param name in the return parameters to avoid this issue.

Fixes #1049

[ericglau]

Note: eventually we may want to remove any NatSpec and comments (other than the NatSpec for the ERC7201 storage location annotations for structs) to help avoid these kinds of issues.