C++: fix constantValue for EnumeratorDeclaration

For the case:
const int x = 12;
enum E { e = x };

constantValue for EnumeratorDeclaration has value=' x'. It should have 'x'.

Change-Id: Iaca77cccd1e0dc5274696b0c96cec6ac2f904979
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>