Rollup merge of #78164 - Aaron1011:fix/async-region-name, r=tmandry

Prefer regions with an `external_name` in `approx_universal_upper_bound`

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.

src/test/ui/async-await/issue-74072-lifetime-name-annotations.stderr

@@ -2,7 +2,7 @@ error[E0506]: cannot assign to `*x` because it is borrowed
   --> $DIR/issue-74072-lifetime-name-annotations.rs:9:5
    |
 LL | pub async fn async_fn(x: &mut i32) -> &i32 {
-   |                                       - let's call the lifetime of this reference `'1`
+   |                          - let's call the lifetime of this reference `'1`
 LL |     let y = &*x;
    |             --- borrow of `*x` occurs here
 LL |     *x += 1;

src/test/ui/async-await/issue-75785-confusing-named-region.rs

@@ -0,0 +1,13 @@
+// edition:2018
+//
+// Regression test for issue #75785
+// Tests that we don't point to a confusing named
+// region when emitting a diagnostic
+
+pub async fn async_fn(x: &mut i32) -> (&i32, &i32) {
+    let y = &*x;
+    *x += 1; //~ ERROR cannot assign to
+    (&32, y)
+}
+
+fn main() {}

src/test/ui/async-await/issue-75785-confusing-named-region.stderr

@@ -0,0 +1,15 @@
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/issue-75785-confusing-named-region.rs:9:5
+   |
+LL | pub async fn async_fn(x: &mut i32) -> (&i32, &i32) {
+   |                          - let's call the lifetime of this reference `'1`
+LL |     let y = &*x;
+   |             --- borrow of `*x` occurs here
+LL |     *x += 1;
+   |     ^^^^^^^ assignment to borrowed `*x` occurs here
+LL |     (&32, y)
+   |     -------- returning this value requires that `*x` is borrowed for `'1`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.