[llvm-rc] Add ACCELERATORS parsing ability. (parser, pt 3/8).

This improves the current llvm-rc parser by the ability of parsing
ACCELERATORS statement.

Moreover, some small improvements to the original parsing commit
were made.

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

Differential Revision: https://reviews.llvm.org/D36894

llvm-svn: 311946