Turns out there's an actual way to specify C++17 for MSBuild purposes
besides just passing the compile flag.
## References
* Future C++20 support (modules)
## PR Checklist
* [x] Closes random fact found while exploring VS16.8 preview C++20
modules.
* [x] I work here.
* [x] It still builds.
## Detailed Description of the Pull Request / Additional comments
* We've been setting C++17 with just the flag passed to the compiler
`cl.exe`. But it turns out that this particular `LanguageStandard`
option will need to be set appropriately one day for us to use C++20
modules (as evidenced by the latest VS16.8 preview that I tried out to
explore modules.) The `AdditionalOption` alone isn't enough to ensure
that modules can be "seen" by other projects after production, but
`LanguageStandard` is (and will set the compiler option as appropriate
as well as whatever internal goo that MSBuild needs to hook up other
stuff.)
## Validation Steps Performed
* Built with it changed.