swift234

Extend the AST walker that performs type coercion to have two modes:
one that performs the coercion on the AST and produces diagnostics for
any problems, and one that simply determines whether the coercion
would succeed. The former already existed (as
TypeChecker::convertToType), while an approximation of the latter was
already implemented as Expr::getRankOfConversionTo(). Unifying these
code paths addresses several issues:

  - Lots of redundancy in the code for computing these two properties,
    which causes bugs. For example, Expr::getRankOfConversionTo()
    wasn't able to handle literals (!).
  - Determining whether a conversion is possible will need the full
    power of the type checker, e.g., to deal with closures in cases
    like:

      func f(g : (int, int) -> int);
      func h() { f({$1 + 0}); }

    Although this is not handled correctly now.

I have opted not to adopt Clang's ImplicitConversionSequence (or
InitializationSequence) design, which computes a record of the steps
needed to perform the conversion/coercion and later applies those
steps, because we don't need that complexity yet. If we start
introducing more implicit conversions into the language, we'll revisit
this decision.



Swift SVN r1308