Added support for persistent variables to the

expression parser.  It is now possible to type:

(lldb) expr int $i = 5; $i + 1
(int) 6
(lldb) expr $i + 2
(int) 7

The skeleton for automatic result variables is
also implemented.  The changes affect:

- the process, which now contains a 
  ClangPersistentVariables object that holds
  persistent variables associated with it
- the expression parser, which now uses
  the persistent variables during variable
  lookup
- TaggedASTType, where I loaded some commonly
  used tags into a header so that they are
  interchangeable between different clients of
  the class

llvm-svn: 110777