Fix purity check for val inside of object

[aherlihy]

Fixes #17317

[smarter]

I was a bit confused by what was going on here, but after reading 471fcd9 it seems the issue is that:

1. Constructors of traits that do not have side-effects will return true to isStableMember
2. Other members might return true just because they're stable (always return the same value) even if they do have side-effects. The previous check excluding lazy was not broad enough to handle this (in fact, arbitrary methods can be marked stable with scala.annotation.unchecked.uncheckedStable)

If I understand 471fcd9 correctly, I think this check is equivalent to:

Suggested change

	|| fn.symbol.isStableMember && fn.symbol.isConstructor // constructors of no-inits classes are stable
	|| fn.symbol.isConstructor && fn.symbol.owner.is(NoInits)

Which is more obviously correct. @sjrd: Can you confirm my understanding here?

[dwijnand]

I think this check is equivalent to:

I don't think so, because the condition required to set the constructor as stable is not just fn.symbol.owner.is(NoInits) but

if myFlags.is(Trait) then myFlags.is(NoInits) else isNoInitsRealClass

where

    def isNoInitsRealClass(using Context): Boolean =
      isRealClass &&
      (asClass.baseClasses.forall(_.is(NoInits)) || defn.isAssuredNoInits(symbol))

So isStableMember is the efficient way to verify that.

Perhaps we can fn.symbol.isStableMember && fn.symbol.isConstructor into a isStableConstructor method, and then throw this detail in its docs. Or we can do that in situ here.