swift234

Sema: Allow an implicit conversion from T? to U? if T can be converted to U.

Similar to the T -> U? conversion, allow T? to be converted to U? through a bind-convert-inject chain. Naively adding this conversion creates ambiguities in the type checker, opening up multiple paths to the same solution in certain cases, so add a a special case to the solver that avoids introducing the conversion if a solution is already available by other means. (Doug figured out that part; thanks Doug!)

This still introduces a regression in test/Constraints/optional.swift, rendering a call ambiguous when overloaded on argument types. <rdar://problem/15367036>

Swift SVN r9857