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

Author: Zalathar

src/tools/tidy/src/issues.txt

@@ -409,7 +409,6 @@ 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

@@ -1,3 +1,9 @@
+//! 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.
+
 //@ run-pass
 //@ edition:2021