[lldb-mi] Simplify MICmnMIOutOfBandRecord implementation.

Summary:
* Remove extraneous members that were just storing temporary
  values.
* OutOfBand_e parameters don't need to be const as they are
  scalars.
* Switch from a map with CMIUtilString values to using a mapping
  function. This uses a switch statement which will generate
  a warning if a new result class is added.
* Make BuildAsyncRecord a static function rather than a private
  member function so that we can construct the result text
  correctly and avoid having extra stuff in the header.

Reviewers: abidh, ki.stfu

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11751

llvm-svn: 243975