Compute the right span for abstract error messages

[mbovel]

Fixes #22941. Done during the compiler spree of September 1st, 2025.

[mbovel]

Future work: should we use the same source position (with the complete span) for other messages in RefChecks.checkAllOverrides?

[mbovel]

Note: there is an existing nameSpan method on trees that implement a similar logic:

scala3/compiler/src/dotty/tools/dotc/ast/Trees.scala

Lines 380 to 389 in fe457a3

	def nameSpan(using Context): Span =
	if (span.exists) {
	val point = span.point
	if (rawMods.is(Synthetic) || span.isSynthetic || name.toTermName == nme.ERROR) Span(point)
	else {
	val realName = srcName.stripModuleClassSuffix.lastPart
	Span(point, point + realName.length, point)
	}
	}
	else span

However it seems we can't use it because we don't have a tree for the class when doing the checks in RefChecks. We therefore copied the logic we needed there.

[tgodzik]

LGTM!