fix <rdar://problem/20167543> "for var x = ..." not parsed as a foreach loop

This was because the ambiguity between c-style and foreach loops wasn't being
properly handled.  Use the canParsePattern() logic to handle this in full 
generality.

Since that logic was unused, dust it off and clean it up a bit.  Similarly,
remove some old vestigates of default argument parsing in tuples and 
old-syntax array handling.



Swift SVN r26164

include/swift/Parse/Parser.h

@@ -1050,7 +1050,6 @@ public:
   bool canParseTypeComposition();
   bool canParseTypeTupleBody();
   bool canParseTypeAttribute();
-  bool canParseTypeArray();
   bool canParseGenericArguments();
 
   bool canParsePatternTuple();