Revert "[perl #79908] Stop sub inlining from breaking closures"

This reverts commit 137da2b05b4b7628115049f343163bdaf2c30dbb. See the
"How about having a recommended way to add constant subs dynamically?"
thread on perl5-porters, specifically while it sucks that we have this
bug, it's been documented to work this way since 5.003 in "Constant
Functions" in perlsub:

    If the result after optimization and constant folding is either a
    constant or a lexically-scoped scalar which has no other references,
    then it will be used in place of function calls made without C<&>

    -- http://perldoc.perl.org/perlsub.html#Constant-Functions

Since we've had this documented bug for a long time we should introduce
this fix in a deprecation cycle rather than silently slowing down code
that assumes it's going to be optimized by constant folding.

I didn't revert the tests it t/op/sub.t, but turned them into TODO tests
instead.

Conflicts:
	t/op/sub.t