Consider optimization removing storage of CommandLineArguments in ProjectSystemProjectOptionsProcessor

Per Jason, it *may* be possible to not store this in certain scenarios. This came up in review of https://github.com/dotnet/roslyn/pull/78446

It is possible (probably not likely) we see an increase in mgd heap in legacy projects due to that PR. It removed hiding those allocations in temporary storage, but kept around the commandline in certain cases instead. _commandLineArgumentsForCommandLine appears to only be stored for a particular scenario that I'll ask Jason to outline.

Start by reviewing pull request 78446 and tracing _commandLineArgumentsForCommandLine in ProjectSystemProjectOptionsProcessor. Identify the particular scenario in which it is stored, then determine whether that storage can be removed without increasing managed heap usage for legacy projects. Done means the scenario and safe optimization are established with supporting validation.

Written by the indexing model from the issue text.