[lldb][NFCI] Remove use of ConstString from IOHandler

None of these need to be in the ConstString StringPool. For the most
part they are constant strings and do not require fast comparisons.

I did change IOHandlerDelegateMultiline slightly -- specifically, the
`m_end_line` member always has a `\n` at the end of it now. This was so
that `IOHandlerGetControlSequence` can always return a StringRef. This
did require a slight change to `IOHandlerIsInputComplete` where we must
drop the newline before comparing it against the input parameter.

Differential Revision: https://reviews.llvm.org/D151597