Simplify enclosesLifetimeOf

[dkorpel]

The fallback on looking at the line and column number was added when sequenceNumber was set in the constructor of a VarDeclaration, but since #13530 it's set in VarDeclaration.semantic. Looking at the line number is unreliable because it can be set with #line directives, so this should be simpler and more robust.

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[thewilsonator]

Needs a rebase to clear CI errors