Remove constant.pm-specific behaviour from Internals::SvREADONLY

Some stuff on CPAN is using this undocumented function, so give
constant.pm its own.  It is already a core module, depending on
functionality provided by the core solely for its sake; so this
does not really change its relationship to the core.
Increase $constant::VERSION to 1.31
lib/diagnostics.t: Use different test data

This is testing that things work for two adjacent =item lines in
perldiag.pod that share the same description.  However a future commit
will change perldiag.pod so that one of the specific adjacent lines
previously used here will be removed.  Therefore, choose a different set
of adjacent lines that aren't scheduled to be changed.

This adds a comment in perldiag that the newly chosen adjacent lines are
thus used in diagnostics.t
lib/diagnostics.t: Add test

This adds a test so that both =item lines of an adjacent pair get
tested.