This is for the 'freestanding' build to stop assuming the platform has argc/argv.
- Introduce a new sub-library, libswiftCommandLineSupport.a
- Move stubs/CommandLine.cpp into this library
- Conditionally embed it into libswiftCore
- Conditionally embed it into libswiftPrivateLibcExtras if not in libswiftCore to support testing
- Add SWIFT_STDLIB_HAS_COMMANDLINE CMake (and build-script) flag