[patch] PR debug/58123: Set correct location for TRY blocks

[Aldy Hernandez <aldyh at redhat dot com>]

[Ughh...I'm apparently incapable of CCing gcc-patches on the first try. 
 My apologies again.]

Please see the long explanation in the PR:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58123

But the short explanation for this patch is that, at the very least, we
should fix the location of the GIMPLE_TRY_FINALLY.  Currently  we're
preferring the input_location, instead of the location of the
TRY_FINALLY_EXPR.

An incorrect GIMPLE_TRY_FINALLY location causes the location of
location-less FINALLY statements to also have an incorrect location,
causing an inconsistent debugging experience.

As explained in the PR, I would ideally like to get rid of the kludge
where we set the location of location-less FINALLY statements to be that
of the TRY statement, but that may be by design, and/or beyond the scope
of this fix.

Tested on x86-64 Linux.

OK for mainline?

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