Extended the IR interpreter to handle the variables

"_cmd", "this", and "self".  These variables are handled
differently from all other external variables used by
the expression.  Other variables are used indirectly
through the $__lldb_arg operand; only _cmd, this, and
self are passed directly through the ABI.

There are two modifications:

 - I added a function to ClangExpressionDeclMap that
   retrives the value of one of these variables by name;
   and

 - I made IRInterpreter fetch these values when needed,
   and ensured that the proper level of indirection is
   used.

llvm-svn: 143065