swift234

Allow implicit conversion constraint to bridge String to NSString even
when it is the favored constraint of a disjunction.

This allows 'ns ?? "str" as String as String' to typecheck where ns is
an NSString. This does not regress the relevant test for 17962491.

<rdar://problem/20029786> Swift compiler sometimes suggests changing "as!" to "as?!"

Swift SVN r25910