[lldb-mi] Use find, not find_first_of, for "--".

Summary:
find_first_of will look for any of the characters, not the full
string passed in. When looking for "--" then, we must use find
and not find_first_of.

Reviewers: ki.stfu, abidh

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12517

llvm-svn: 246529