Add new 'CXXConditionDeclExpr' expression node used for a 'condition'...

Add new 'CXXConditionDeclExpr' expression node used for a 'condition' declaration, e.g: "if (int x=0) {...}".
It is a subclass of DeclRefExpr and the main difference is that CXXConditionDeclExpr owns the declaration that it references.

llvm-svn: 56033