swift234

Add _isFast predicate that is true at Ofast

We want to support three configurations:

* Debug (-Onone, -O0): user assertions, library precondition checks, runtime
  checks enabled and verbose.

* Release (-O): library precondition checks, runtime checks enabled but succinct
  (trap).

* Fast (-Ofast): all checks off.

The _isFast predicate will allow to write standard library functions to support
this plan. This commit changes fatal() to differentiate between the three modes.

Support for rdar://16477198

Swift SVN r17697