[CommandLine] Remove OptionCategory and SubCommand caches from the Option class.

Summary:
This change processes `OptionCategory`s and `SubCommand`s as they
 are seen instead of caching them in the Option class and processing
them later.  Doing so simplifies the work needed to be done by the Global
parser and significantly reduces the size of the Option class to a mere 64
bytes.

Removing  the `OptionCategory` cache saved 24 bytes, and removing
the `SubCommand` cache saved an additional 48 bytes, for a total of a
72 byte reduction.

Reviewers: beanz, zturner, MaskRay, serge-sans-paille

Reviewed By: serge-sans-paille

Subscribers: serge-sans-paille, tstellar, zturner, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62105

llvm-svn: 364134