swift234

Add runtime integer truncation checking to the conversion constructors

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