swift234

Implement overflow checking on integer literals (1 of 2)

- Added 2 new builtins strunc_with_overflow and utrunc_with_overflow
  that perform truncation and produce a compile time error when truncation
  overflows.

- Used these builtins instead of trunc to implement "_convertFromBuiltinIntegerLiteral".

- Currently, the builtins are converted to trunc in IRGen, but we should
  not be IRGenning code that uses them, since all uses of
  "_convertFromBuiltinIntegerLiteral" should be inlined and the arguments
  constant folded.

- I had to change a test and the implementation of operator '~' in the standard library
  because they assumed that '0xFF' is a valid signed Int8. It is questionable if we should
  allow this and if we should treat signed and unsigned integers differently depending on
  how they are spelled (decimal or hexadecimal).

* This patch will be further improved (Ex: will start finding overflows on Int64, better
  deal with '-128' after the negative integer literal patch is committed.)

Swift SVN r9226