* Add support for null as a constant

* Add support for null as a constant
* Allow multiple definitions of a type with the same name as long as they are the same type
* Eagerly resolve types to allow #2 to work instead of after the whole const pool has been processed
* Change grammar to require a const before a local constant definition

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@699 91177308-0d34-0410-b5e6-96231b3b80d8