BUG-576: added support for handling nested groupings in code generator.

Failing example (causing NullPointerException):

container foo {
  grouping bar {}
  uses bar;
}

Added call to resolve groupings before resolving uses.

Change-Id: I171b3bbdd26feaf67c3488a55e7d9e3b6ec4a915
Signed-off-by: Martin Vitez <mvitez@cisco.com>
Merge "BUG-576: added support for handling nested groupings in code generator."