Add a flag to make Apple rules not share C++ compile actions.

This is accomplished by:

- Setting the APPLE_CROSSTOOL configuration distinguisher in AppleCrosstoolTransition
- Doing nothing in AppleCrosstoolTransition in case we are already in an Apple configuration so that funky use cases like a binary having a dependency on a swift_library both directly and through an objc_library work
- Adding the "apl-" prefix to the output directory name in APPLE_CROSSTOOL configurations

Plus a few minor cleanups:

- Removed some unused methods
- Nopped out --enable_apple_crosstool_transition if the new flag is set
- Nopped out --target_uses_apple_crosstool if the new flag is set

These latter reduce the possible space of Apple configurations, thus making the code a bit more comprehensible.

RELNOTES: None.
PiperOrigin-RevId: 206157413

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