CXXRecordDecl and RecordDecl:

- Change constructor and create methods to accept a CXXRecordDecl* (RecordDecl*)
  instead of a ScopedDecl* for PrevDecl.  This causes the type checking
  to be more tight and doesn't break any code.
  
RecordDecl:

- Don't use the NextDeclarator field in ScopedDecl to represent the previous
  declaration. This is a conflated use of the NextDeclarator field, which will
  be removed anyway when DeclGroups are fully implemented.

- Instead, represent (a soon to be implemented) chain of RecordDecls using a
  NextDecl field.  The last RecordDecl in the chain is always the 'defining'
  RecordDecl that owns the FieldDecls.  The other RecordDecls in the chain
  are forward declarations.

llvm-svn: 55640