InputCommon/ExpressionParser: Function argument parsing minor cleanup.

members instead of passing arguments.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

There's nothing within this function that requires a copy of the string
to be made, so we can make use of a non-owning view

This header was including itself, which is likely not intended.

Since we target C++17, we can collapse the namespaces into a single
declaration specifier.