Rename test and update docstring; it's not a closure

Suggested by @lcnr in code review.

Author: amandasystems

tests/ui/borrowck/static-trait-bound-lost-closure.rs renamed to tests/ui/borrowck/static-trait-bound-lost.rs

@@ -1,6 +1,6 @@
 // This test is a reduced version of a bug introduced during work on type-tests for Polonius.
-// The underlying problem is that the 'static bound is lost for a closure that is threaded
-// deeply enough, causing an error.
+// The underlying problem is that the 'static bound is lost for a type parameter that is
+// threaded deeply enough, causing an error.
 // The bug was first observed in exr-1.4.1/src/image/read/mod.rs:124:5 during perf test.
 
 //@ check-pass