swift234

SIL: Start on a StripRuntimeChecks pass.

In the long term we want more detailed configurability of runtime checks, but for our short-term performance work we just want a blanket on/off switch. Add a StripRuntimeChecks SIL pass that, as a start, converts invocations of checked overflow builtins to the equivalent unchecked builtins and kills cond_fails. Expose it through the compiler with a -disable-all-runtime-checks switch.

NB: I haven't tested building the stdlib or running the tests with the switch thrown yet.

Swift SVN r12379