dead_branch_remover removes PURE annotations on sequence expressions

[devongovett]

In code like /*#__PURE__*/ (0, forwardRef)(Foo);, the dead_branch_remover pass will simplify this to forwardRef(Foo) and remove the PURE annotation which might be used in later passes. I added a test to show this, and commented out the line that causes it. However I'm not quite sure how to fix it.

I guess the comment is attached to the original sequence expression which was removed. So we need to move the comment to the resulting expression somehow, or maybe change the span of the expression to match the old one.

I tried passing in the comments to this pass so I could move it, but it was a bit hard because of the Parallel implementation that would require the comments to be Send + Sync.

Any suggestions on the best approach here?

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[kdy1]

You can

1. assign the span of the node with PURE comment to the emitted node

2. Accept C: Comments or &dyn Comments and call move_leading