Cling's error recovery has to be able to revert declarations that are declared

multiple times (aka redeclarable declarations). When clang sees redeclaration 
it creates a single-linked list, which head points to the last declaration of 
the list. Consider the example:
typedef int A;
typedef int B;
typedef int C;
extern A i;
extern B i;
extern C i;
int i;

That would create the following redeclaration chain:

 +---+    +---+    +---+    +---+
 | A | <- | B | <- | C | <- | i |
 +---+    +---+    +---+    +---+
   |                          ^
   |                          |
   +--------------------------+

In order to remove the last declaration we need to rebuild the redeclaration chain.

* Rebuild properly the redeclaration chain
* Add tests for that
* Improve the current error recovery infrastructure - this will be changed a lot soon.


git-svn-id: http://root.cern.ch/svn/root/trunk@41851 27541ba8-7e3a-0410-8455-c3a389f83636