swift234

Two changes:
 - Improve handling of if_expr in a couple of ways: teach constraint simplification
   about IfThen/IfElse and teach CSDiags about the case when the cond expr doesn't match
   BooleanType.  This is rarely necessary, but CSDiags is all about cornercases, and this
   does fix a problem in a testcase.
 - Be a bit more specific about the constraint failure kind (e.g. say subtype) and when
   we have a protocol conformance failure, emit a specific diagnostic about it, instead of
   just saying that the types aren't convertible.




Swift SVN r30650