Restrict use of default arguments on protocol method signatures.

We were never handling this correctly, and default arguments are checked along with the method body.
In some cases where no further validation was necessary (such as when the default argument was a literal value),
the compiler would let this slip through but in others this would cause a crash.  (rdar://problem/16476405)

Swift SVN r15736

lib/Parse/ParsePattern.cpp

@@ -89,6 +89,8 @@ static ParserStatus parseDefaultArgument(Parser &P,
     auto inFlight = P.diagnose(equalLoc, diag::non_func_decl_pattern_init);
     if (initR.isNonNull())
       inFlight.fixItRemove(SourceRange(equalLoc, initR.get()->getEndLoc()));
+  } else {
+    defaultArgs->HasDefaultArgument = true;
   }
 
   if (initR.hasCodeCompletion()) {