[CommandLine] Allow grouping options which can have values.

This patch allows all forms of values for options to be used at the end
of a group. With the fix, it is possible to follow the way GNU binutils
tools handle grouping options better. For example, the -j option can be
used with objdump in any of the following ways:

$ objdump -d -j .text a.o
$ objdump -d -j.text a.o
$ objdump -dj .text a.o
$ objdump -dj.text a.o

Differential Revision: https://reviews.llvm.org/D58711

llvm-svn: 355185