New feature: OptionList.

It can be handy to have all information about options gathered in a single place
to provide an overview of all supported options. This patch allows the following:

def Options : OptionList<[
(switch_option "E", (help "Help string")),
(alias_option "quiet", "q")
...
]>;

Tool-specific option properties (like 'append_cmd') have (obviously) no meaning in
this context, so the only properties that are allowed are 'help' and 'required'.

See usage example in examples/Clang.td.

llvm-svn: 51754