ClangExpressionVariable previously was not capable of
handling multiple clients.  However, occasionally an
expression must be run in the service of another
expression, and in this case two parsers need to access
the same list of persistent variables.

To allow this, persistent variables now provide state
for multiple parsers, and parsers must allocate, access,
and deallocate this state by providing their own ID
(at the moment, simply the value of the "this" pointer).

<rdar://problem/12914539>