[Parser] Preserve try labels (#6505)

In the standard text format, try scopes can be targeted by both normal branches
and delegates, but in Binaryen IR we only allow them to be targeted by
delegates, so we have to translate branches to try scopes into branches to
wrapper blocks instead. These wrapper blocks must have different names than the
try expressions they wrap, so we actually need to track two label names for try
expressions: one for delegates and another for normal branches.

We previously tried to avoid this complexity by tracking only the branch label
and computing the delegate label from the branch label as necessary, but that
produced unnecessary wrapper blocks and ugly label names that did not appear in
the source.

To produce better IR and minimize the diff when switching to the new text
parser, bite the bullet and track the delegate and branch label names
separately. This eliminates unnecessary wrapper blocks and keeps try names the
same as in the wat source where possible.

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