[conv.integral, conv.double] Fix promotion conversions

[sdkrystian]

• Instead of delegating integral conversions to bool to [conv.bool], we can just exclude them from the set of integral conversions (no normative effects as it delegates).
• [conv.integral] p2 & p3 can be merged as the work better as a single paragraph (due to "Otherwise [...]").
• There is only one floating-point promotion conversion (from float to double); we need not say "conversions".

[jensmaurer]

These seem to be three independent changes. Please make that three commits.

[sdkrystian]

@jensmaurer Should be good now.

[zygoloid]

This appears to be a normative change: it affects which conversions are permitted in converted constant expressions. It might be a good change, and it's certainly clearer to have each conversion have only one kind, but this should probably be handled by a core issue.

[tkoeppe]

@sdkrystian Could you please rebase?

[sdkrystian]

@tkoeppe Yup, will do it Monday

[sdkrystian]

@tkoeppe Rebased

[tkoeppe]

Thanks! We'll have to look at it in Core, but we wanted a clean diff.