Allow ${^WARNING_BITS} to turn off lexical warnings

Allow ${^WARNING_BITS} to turn off lexical warnings

Various magical modules copy hints from one scope to another.  But
copying ${^WARNING_BITS} doesn’t always copy the same hints.  If lexi-
cal warnings are not on at all, ${^WARNING_BITS} returns a different
value depending on the current value of $^W.  Setting ${^WARNING_BITS}
to its own value when $^W is true will stop $^W from being able to
control the warnings in the current compilation scope.  Setting
${^WARNING_BITS} to its own value when $^W is false causes even
default warnings to be suppressed.

This commit makes undef a special value that represents the default
state, in which $^W controls warnings.