[CommandLine] Enable Grouping for short options by default. Part 4 of 5

Summary:
This change enables `cl::Grouping` for short options --
options with names of a single character.  This is consistent with GNU
getopt behavior.

Reviewers: rnk, MaskRay

Reviewed By: MaskRay

Subscribers: thopre, cfe-commits, MaskRay, rupprecht, hiraditya, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D61270

llvm-svn: 359917