When failing to find a suitable type witness, always record ErrorType.

Fixes the crash in rdar://problem/20564378. In these cases, we end up
swallowing some diagnostics. That will be addressed in a subsequent
commit.

Swift SVN r27436

lib/Sema/CSApply.cpp

@@ -5199,6 +5199,10 @@ Expr *ExprRewriter::convertLiteral(Expr *literal,
     if (!argType)
       return nullptr;
 
+    // If the argument type is in error, we're done.
+    if (argType->is<ErrorType>())
+      return nullptr;
+
     // Convert the literal to the non-builtin argument type via the
     // builtin protocol, first.
     // FIXME: Do we need an opened type here?