compiler: Report errors for malformed builtin calls.

Errors reported from malformed builtin calls are handled in a later
pass than the one in which they are detected.  If a malformed builtin
call is lowered into an error expression too early, these errors will
never be reported.

Fixes golang/go#11561.

Change-Id: I9c92e92445eb9206b8f4b44949bb126209a82b31
Reviewed-on: https://go-review.googlesource.com/12778
Reviewed-by: Ian Lance Taylor <iant@golang.org>

go/expressions.cc

@@ -8499,7 +8499,8 @@ Call_expression::do_lower(Gogo* gogo, Named_object* function,
     {
       if (!this->fn_->type()->is_error())
 	this->report_error(_("expected function"));
-      return Expression::make_error(loc);
+      this->set_is_error();
+      return this;
     }
 
   // Handle an argument which is a call to a function which returns