Add regression test

Author: bjorn3

tests/ui/parallel-rustc/cycle_crash.rs

@@ -0,0 +1,6 @@
+//@ compile-flags: -Z threads=2
+//@ check-fail
+
+const FOO: usize = FOO; //~ERROR cycle detected when simplifying constant for the type system `FOO`
+
+fn main() {}

tests/ui/parallel-rustc/cycle_crash.stderr

@@ -0,0 +1,18 @@
+error[E0391]: cycle detected when simplifying constant for the type system `FOO`
+  --> $DIR/cycle_crash.rs:4:1
+   |
+LL | const FOO: usize = FOO;
+   | ^^^^^^^^^^^^^^^^
+   |
+note: ...which requires const-evaluating + checking `FOO`...
+  --> $DIR/cycle_crash.rs:4:20
+   |
+LL | const FOO: usize = FOO;
+   |                    ^^^
+   = note: ...which again requires simplifying constant for the type system `FOO`, completing the cycle
+   = note: cycle used when running analysis passes on this crate
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0391`.