Remove "NextDecl" from RecordDecl.  This change touches many files that where RecordDecl or CXXRecordDecl was constructed, always with an argument of 'NULL' for the previous declaration.

The motivation behind this change is that chaining the RecordDecls is simply unnecessary.  Once we create multiple RecordDecls for the same struct/union/class, clients that care about all the declarations of the same struct can build a back map by seeing which Decls refer to the same RecordType.

llvm-svn: 55821