Skip warnable const folding outside warnings scope

Constant folding is not supposed to warn.  If it would produce a warn-
ing, then it is skipped and the warning is deferred to run time.

This means the -w flag can affect constant folding:

$ ./perl -Ilib -MO=Concise -le 'use constant u=>undef; $a = u+1'
6  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 132 -e:1) v:{ ->3
5     <2> sassign vKS/2 ->6
3        <$> const[NV 1] s/FOLD ->4
-        <1> ex-rv2sv sKRM*/1 ->5
4           <#> gvsv[*a] s ->5
-e syntax OK
$ ./perl -Ilib -MO=Concise -lwe 'use constant u=>undef; $a = u+1'
8  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 132 -e:1) v:{ ->3
7     <2> sassign vKS/2 ->8
5        <2> add[t2] sK/2 ->6
3           <$> const[NULL ] s*/FOLD ->4
4           <$> const[PVNV 1] s ->5
-        <1> ex-rv2sv sKRM*/1 ->7
6           <#> gvsv[*a] s ->7
-e syntax OK

But the problem here is that the flag could be turned on at run time,
so if the folding happens because -w is off, then the behaviour
changes due to folding.  It’s fine to do the folding here only when
warnings are lexically disabled, as that overrides any setting of -w.