Rename test to unresolvable-upvar-issue-87987.rs and add some notes

Author: Zalathar

src/tools/tidy/src/issues.txt

@@ -410,7 +410,6 @@ ui/closure_context/issue-26046-fn-once.rs
 ui/closure_context/issue-42065.rs
 ui/closures/2229_closure_analysis/issue-118144.rs
 ui/closures/2229_closure_analysis/issue-87378.rs
-ui/closures/2229_closure_analysis/issue-87987.rs
 ui/closures/2229_closure_analysis/issue-88118-2.rs
 ui/closures/2229_closure_analysis/issue-88476.rs
 ui/closures/2229_closure_analysis/issue-89606.rs

tests/ui/closures/2229_closure_analysis/issue-87987.rs renamed to tests/ui/closures/2229_closure_analysis/unresolvable-upvar-issue-87987.rs

@@ -1,6 +1,12 @@
 //@ run-pass
 //@ edition:2021
 
+// When a closure syntactically captures a place, but doesn't actually capture
+// it, make sure MIR building doesn't ICE when handling that place.
+//
+// Under the Rust 2021 disjoint capture rules, this sort of non-capture can
+// occur when a place is only inspected by infallible non-binding patterns.
+
 struct Props {
     field_1: u32, //~ WARNING: fields `field_1` and `field_2` are never read
     field_2: u32,

tests/ui/closures/2229_closure_analysis/issue-87987.stderr renamed to tests/ui/closures/2229_closure_analysis/unresolvable-upvar-issue-87987.stderr

@@ -1,5 +1,5 @@
 warning: fields `field_1` and `field_2` are never read
-  --> $DIR/issue-87987.rs:5:5
+  --> $DIR/unresolvable-upvar-issue-87987.rs:11:5
    |
 LL | struct Props {
    |        ----- fields in this struct