Don't call `Warning.warn` unless the category is enabled (#10981)

	Don't call `Warning.warn` unless the category is enabled

	The warning category should be enabled if we want to call
	`Warning.warn`.

	This commit speeds up the following benchmark:

	```ruby
	eval "def test; " +
	  1000.times.map { "'  '.chomp!" }.join(";") + "; end"

	def run_benchmark count
	  i = 0
	  while i < count
	    start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
	    yield
	    ms = Process.clock_gettime(Process::CLOCK_MONOTONIC) - start
	    puts "itr ##{i}: #{(ms * 1000).to_i}ms"
	    i += 1
	  end
	end

	run_benchmark(25) do
	  250.times do
	    test
	  end
	end
	```

	On `master` this runs at about 92ms per iteration. With this patch, it
	is 7ms per iteration.

	[Bug #20573]