[cmake] Set SWIFT_STDLIB_HAS_COMMANDLINE

[etcwilde]

Set the SWIFT_STDLIB_HAS_COMMANDLINE macro when the stdlib has command line support. This is needed while building SwiftCore to ensure that it exports the appropriate symbols.

[etcwilde]

@swift-ci please smoke test

[compnerd]

I wonder if we can handle this slightly differently. SwiftPrivateLibcExtras also needs this flag. The shared options are going to be interesting as we need the rest of the modules integrated before we can start hoisting these to avoid having them diverge in different places.

[etcwilde]

The macro is public, so linking against it should propagate it. Once we're to the install story and have CMake export the targets for the supplemental libraries to consume, I believe we can mark the link public on swiftCore, and then linking against swiftCore will pick up the define appropriately too. Either way, we should be able to include it in the config file that the overlay and supplemental libraries consume.