[CommandLine] Do not crash if an option has both ValueRequired and Grouping.

If an option, which requires a value, has a `cl::Grouping` formatting
modifier, it works well as far as it is used at the end of a group,
or as a separate argument. However, if the option appears accidentally
in the middle of a group, the program just crashes. This patch prints
an error message instead.

Differential Revision: https://reviews.llvm.org/D58499

llvm-svn: 355184