[lldb-mi] Clean up CMICmdArgSet usage.

Summary:
CMICmdArgSet stores a vector of non-const pointers to the arguments
that it is validating. It owns them and is responsible for deleting
them.

We don't need to pass a const reference to the argument to
CMICmdArgSet::Add and then take the address and const_cast it
when we can just pass the argument pointer in directly.

This lets us remove some noise at every call site for CMICmdArgSet::Add
and then clean up a couple of bits inside CMICmdArgSet to remove
const_casts.

Reviewers: abidh, ki.stfu, domipheus

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12878

llvm-svn: 247677