Fix Issue 20542 - std.math.nextafter(NaN, y) and nextafter(x, NaN) sh…

[e10s]

…ould return NaN

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20542	normal	std.math.nextafter(NaN, y) and nextafter(x, NaN) should return NaN

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7380"

[ghost]

When I read #7176 correctly, this unittest was commented out on purpose by @n8sh, as it might not work on every real implementation yet.

[e10s]

It doesn't matter in these special cases as long as realX == realY and isNaN(realVal) work properly at compile time. #7176 just mentions nextUp / nextDown for real, called if both of the arguments are different and not NaN.

[n8sh]

I didn't need to comment out those two lines since typeof does not require the contained expression to run in CTFE.

[thewilsonator]

Sorry there has been some network problems, could you please force-push to restart the tests, thanks.