Support command line option categories.

Patch by Dan Liew!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181253 91177308-0d34-0410-b5e6-96231b3b80d8
Implemented public interface for modifying registered (not positional or sink options) command line options at runtime.

Patch by Dan Liew!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181254 91177308-0d34-0410-b5e6-96231b3b80d8