Fix issues with C "tentative" definitions.

- Move checking from MergeVarDecl->FinializeDeclaratorGroup. Since MergeVarDecl is called before the initializer is attacted, it can't be done there (this removes a long standing FIXME).
- Add Sema::isTentativeDefinition() and Sema::CheckForFileScopedRedefinitions().
- Remove FIXME's and touch-up test case.

Still some more work to do (forthcoming)...

llvm-svn: 54533