CXXRecordDecl and RecordDecl:

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@55640 91177308-0d34-0410-b5e6-96231b3b80d8