[llvm-rc] Serialize ACCELERATORS to .res files (serialization, pt 2).

This allows llvm-rc to serialize ACCELERATORS resources.

Additionally, as this is the first type of resource to support basic
optional resource statements (LANGUAGE, CHARACTERISTICS, VERSION),

ACCELERATORS statement documentation:
msdn.microsoft.com/en-us/library/windows/desktop/aa380610.aspx
Accelerator table structure documentation:
msdn.microsoft.com/en-us/library/windows/desktop/ms648010.aspx
Optional resource statement fields are described in:
msdn.microsoft.com/en-us/library/windows/desktop/ms648027.aspx

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

Differential Revision: https://reviews.llvm.org/D37824

llvm-svn: 314549