"Unused glint-expect-error" diagnostics sometimes span more source code text than they should

[machty]

Minor issue, but after fixing some glint-expect-error issues in #795, I noticed that the diagnostic span of text for "unused glint-expect-error" directives is a bit too wide. This issue may self-correct if we refactor/simplify some of the ways that Volar maps between source and generated code.