Prefer regions with an external_name in approx_universal_upper_bound

[Aaron1011]

Fixes #75785

When displaying a MIR borrowcheck error, we may need to find an upper
bound for a region, which gives us a region to point to in the error
message. However, a region might outlive multiple distinct universal
regions, in which case the only upper bound is 'static

To try to display a meaningful error message, we compute an
'approximate' upper bound by picking one of the universal regions.
Currently, we pick the region with the lowest index - however, this
caused us to produce a suboptimal error message in issue #75785

This PR approx_universal_upper_bound to prefer regions with an
external_name. This causes us to prefer regions from function
arguments/upvars, which seems to lead to a nicer error message in some
cases.