swift234

[sil-combine] Teach the optimizer how to optimize bridged casts.

If a conformance to _BridgedToObjectiveC is statically known, generate a more efficient code by using the newly introduced library functions for bridging casts.
This covers the casts resulting from SIL optimizations.

Tests are included. I tried to cover most typical casts from ObjC types into Swift types and vice versa and to check that we always generate something more efficient than a checked_cast or unconditional_checked_cast. But probably even more tests should be written or generated by means of gyb files to make sure that nothing important is missing.

The plan is to make the bridged casts SIL optimization a guaranteed optimization. Once it is done, there is no need to lower the bridged casts in a special way inside Sema, because they all can be handled by the optimizer in a uniform way. This would apply to bridging of Error types too.

With this change, no run-time conformance checks are performed at run-time if conformances are statically known at compile-time.
As a result, the performance of rdar://19081345 is improved by about 15%. In the past, conformance checks in this test took 50% of its execution time, then after some improvements 15% and now it is 0%, as it should be.

Swift SVN r27102