[llvm-rc] Refactoring needed for ACCELERATORS and MENU resources.

This is a part of llvm-rc serialization patch set (serialization, pt 1.5).

This:

* Unifies the internal representation of flags in ACCELERATORS and MENU
   with the corresponding representation in .res files (noticed in
   https://reviews.llvm.org/D37828#inline-329828).
* Creates an RCResource subclass, OptStatementsRCResource, describing
   resource statements that can declare resource-local optional statements
   (proposed in https://reviews.llvm.org/D37824#inline-329775).

These modifications don't fit to any of the current patches, so I'm
submitting them as a separate patch.

Differential Revision: https://reviews.llvm.org/D37841

llvm-svn: 314541