Fix #18531 - Disallow (dis)equality with FP.nan/FP.init literals

[0-v-0]

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Testing this PR locally

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[adamdruppe]

does still allow v is float.nan?

edit: lol i see it in the error message but the test should confirm it has no error anyway

[Herringway]

Shouldn't the error mention the actual constants instead of what appears to be internal names like nanF?

[xoxorwr]

Suggesting a Phobos dependency in a compiler error is undesirable imo.
It should probably live in core package.

[dkorpel]

Shouldn't the error mention the actual constants instead of what appears to be internal names like nanF?

This PR uses the correct toErrMsg, it looks like a separate issue: #23270