Fairly large "cleaup" related to changing ObjCCompatibleAliasDecl superclass...

Fairly large "cleaup" related to changing ObjCCompatibleAliasDecl superclass (to inherit from NamedDecl, instead of ScopedDecl). 

- Added a DenseMap to associate an IdentifierInfo with the ObjCCompatibleAliasDecl.
- Renamed LookupScopedDecl->LookupDecl and changed it's return type to Decl. Also added lookup for ObjCCompatibleAliasDecl's.
- Removed Sema::LookupInterfaceDecl(). Converted clients to used LookupDecl().
- Some minor indentation changes.

Will deal with ObjCInterfaceDecl and getObjCInterfaceDecl() in a separate commit...

llvm-svn: 49058