This is different in Ruby 3.0, which appears to deduplicate and
return a new string always. The 3.0 behavior will need to be
preserved in JRuby 9.4.

We use the same table to store the infrequently used object_id,
FFI native pointer reference, and ObjectSpace weakref-manager
objects. When these are the only variables on a given type, any
allocated ivar table will only be large enough to contain these
values. However when the type in question has had a Ruby instance
variable assigned anywhere in the system, all future variable
tables will be sized to handle both the internal variable and the
instance variables, whether or not the instance variables have
been set. This results in future objects looking like they have
instance variables even if they only have internal variables.

This change scans the variable table list, skipping internal
variables, to see if any instance variables have actually been
set. This allows the instance variable check for interned strings
to work properly when some other string in the system received an
instance variable.

* Check for tainting during deduplication and skip interning if
  tainted.

Fixes jruby#7018

Backported to 9.2 from 9.3 via 59f08d5
minus hash key deduplication not present in Ruby 2.5.