Style/WhileUntilModifier must copy logic from Style/IfUnlessModifier

[marcandre]

There's a lot of logic in IfUnlessModifier that is missing from WhileUntilModifier.

1. Looses comments:

while bar # important comment about to be nuked
  baz
end
# corrected to
baz while bar

2. Incorrect correction for assignment:

foo = while bar
  baz
end
# corrected to
foo = baz while bar

This is logic that IfUnlessModifier has.

It should be moved into their shared module, and the corresponding tests should be moved in their shared context.

3. Doesn't detect inline cases:

while foo; bar; end # => no offense

Again, IfUnless does the right thing. Same logic should be applied, tests should be moved in their shared context.

Found while checking #8273. See also #8282 and #8283

[marcandre]

See also #8499