[build-script] Add --skip-build-sourcekit

[modocache]

Expose the CMake SWIFT_BUILD_SOURCEKIT option at the build-script and build-script-impl layer. This allows SourceKit builds to be enabled/disabled via the command-line:

utils/build-script --skip-build-sourcekit

Since SourceKit only builds on macOS at the moment, automatically add --skip-build-sourcekit to non-macOS host builds.

Resolves SR-2653.

[gribozavr]

Thanks for your PR, but this is not exactly how --skip-* flags are supposed to work. --skip-build-* is supposed to not interfere with the configuration step, and just skip the build step. For example, you should be able to run just the configuration step by specifying all --skip-* flags.

I'd suggest that instead you add a --sourcekit flag, and allow specifying a 0 argument to disable it. This will be consistent with --lldb, --swiftpm etc.

[modocache]

I sent this because I thought it was needed for building the runtime for Android on macOS, but I managed to be able to do that with #4972. I still think being able to flip SourceKit on or off would be useful, but I'll close this for now.