[Perl/perl5] c947c5: Use dNOOP for otherwise empty declarations

[Karl Williamson]

  Branch: refs/heads/smoke-me/khw-decl
  Home:   https://github.com/Perl/perl5
  Commit: c947c584aa272eae999dde1cdb52bfeb87c0d902
      
https://github.com/Perl/perl5/commit/c947c584aa272eae999dde1cdb52bfeb87c0d902
  Author: Karl Williamson <k...@cpan.org>
  Date:   2020-04-25 (Sat, 25 Apr 2020)

  Changed paths:
    M perl.h
    M regcomp.c
    M regcomp.h

  Log Message:
  -----------
  Use dNOOP for otherwise empty declarations

Otherwise, the semi-colon the code has afterwards can be interpreted by
some compilers as ending the block of declarations, and treat any later
declarations in the same block as erors in C89.

For an example, see the ticket this fixes #17725