[CommandLine] Reduce size of Option class

Summary:
Reduce size of Option class from 184 bytes to 136 bytes by
placing more member variables in Bit Field (16 bytes), and
reducing the initial sizes of Categories and Subs to 1 (32 bytes).

Saves about 48k for bin/opt.

Reviewed By: beanz

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62091

llvm-svn: 361107