swift234

Add more support for various safety checks.

_precondition_safety_check

  The check is enabled in debug and release mode. In debug mode it prints a
  useful error message. In release mode it traps. In fast mode this is a noop.

_partial_safety_check

  The check is only enabled in debug mode (-Onone).

_internal_library_check

  The check is only enabled when the standard library is compiled with
  "SWIFT_STDLIB_INTERNAL_CHECKS" set to on/yes. It is a noop if the above option
  is not specified.

rdar://16477198

Swift SVN r17789