Add new builtins(by generalizing, renaming, and extending the builtins used for compile time integer literal checking). These new builtins truncate integers and check for overflow/truncation errors at runtime. Use these for FixedPoint conversion constructors.
Fix a routine in stdlib's String implementation and a test that relied on bitwise behavior of the constructors (and triggered overflows).
TODO:
- Teach CCP about these to get static checking.
- Add special builtins for same size signed <-> unsigned conversions.
Swift SVN r10432
