Added automatically generated result variables for each

expression.  It is now possible to do things like this:

(lldb) expr int $i = 5; $i + 1
$0 = (int) 6
(lldb) expr $i + 3
$1 = (int) 8
(lldb) expr $1 + $0
$2 = (int) 14

As a bonus, this allowed us to move printing of
expression results into the ClangPersistentVariable
class.  This code needs a bit of refactoring -- in
particular, ClangExpressionDeclMap has eaten one too
many bacteria and needs to undergo mitosis -- but the
infrastructure appears to be holding up nicely.

llvm-svn: 110896