Made the following changes:

* ValueHolder became a 3 argument template.  This allows for BasicBlock to
  use the value holder arg as a typesafe parent pointer.
* SymTabValue no longer inherits from Value
* Method does not inherit from only SymTabValue.  Now it inherits from both
  STV & Value.
* Module does not inherit from only SymTabValue.  Now it inherits from both
  STV & Value.
* Updated the SymTabValue.h file to reference SymTabValue instead of STDef
  in several places
* Added isArraySelector & isStructSelector to GetElementPtr instruction

llvm-svn: 177