swift234

Allow "isa" patterns to downcast collections and bridge through Objective-C classes.

When checking an isa pattern that requires either collection
downcasting or bridging through an Objective-C class (e.g., 
"is String" or "is Dictionary<String, Int>"), form a conditional
downcast and place it in an expression pattern.

With this change, we can test for these cases (with "is") but we can't
capture the value produced on success (e.g., for "let str as
String"). This is a first small step toward <rdar://problem/17408934>.


Swift SVN r19070