[Diagnostics] Diagnose comparisons with '.nan' and suggest using '.isNan' instead

[theblixguy]

This is based on a suggestion from @stephentyrone on Twitter: https://twitter.com/stephentyrone/status/1302702935831384071

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It is a pretty common mistake to compare values to the .nan static property, instead of using the .isNaN instance property when checking for the presence or absence of NaN.

According to isNaN documentation:

Because NaN is not equal to any value, including NaN, use this property instead of the equal-to operator (==) or not-equal-to operator (!=) to test whether a value is or is not NaN.

So, diagnose comparisons using ==, >= and etc when either of the arguments is .nan from FloatingPoint protocol. For example:

2.0 >= .nan // warning

let value = 0.1
value == .nan // warning

and suggest using isNaN when applicable.

[stephentyrone]

I'm fine with the diagnostics generated. Pavel or Holly may have feedback on the implementation details, so please let one of them review as well.

[theblixguy]

Thank you! While I wait for them to take a look as well, let me run some tests:

@swift-ci please smoke test

[xwu]

This is a good warning to have! Thanks for tackling it.

[theblixguy]

Heh, I didn't notice I wrote isNan instead of isNaN (quite subtle!). Thanks @xwu!

[theblixguy]

@swift-ci please smoke test

[xwu]

@swift-ci please smoke test macOS platform

[xedin]

LGTM! I have left a couple of improvement suggestions inline.

[theblixguy]

@swift-ci please smoke test

[theblixguy]