This allows MENU resources to be serialized.
MENU resource statement doc: msdn.microsoft.com/en-us/library/windows/desktop/aa381025.aspx
POPUP sub-statement doc: msdn.microsoft.com/en-us/library/windows/desktop/aa381030.aspx
MENUITEM sub-statement doc: msdn.microsoft.com/en-us/library/windows/desktop/aa381024.aspx
MENUHEADER structure: msdn.microsoft.com/en-us/library/windows/desktop/ms648018.aspx (and NORMALMENUITEM, POPUPMENUITEM structs).
Thanks for Nico Weber for his original work in this area.
I don't think you need to separate the .rc file from the FileCheck directives for these tests. You can embed comments in the .rc file with //. However, in order for lit to treat a .rc file as a test, you will need to create a lit.local.cfg file that does something like config.suffixes.append('.rc'). You can search the codebase for other examples of this.
This is probably best implemented as a follow-up change, though.