[llvm-rc] Add MENU parsing ability (parser, pt 4/8).

This extends llvm-rc parsing tool by MENU resource
(msdn.microsoft.com/en-us/library/windows/desktop/aa381025(v=vs.85).aspx).
As for now, MENUEX
(msdn.microsoft.com/en-us/library/windows/desktop/aa381023(v=vs.85).aspx)
seems unnecessary.

Thanks for Nico Weber for his original work in this area.

Differential Revision: https://reviews.llvm.org/D36898

llvm-svn: 311956