Add test for type alias mutual recursion

Author: camelid

src/test/ui/infinite/infinite-type-alias-mutual-recursion.rs

@@ -0,0 +1,6 @@
+type X1 = X2;
+//~^ ERROR cycle detected when expanding type alias `X1`
+type X2 = X3;
+type X3 = X1;
+
+fn main() {}

src/test/ui/infinite/infinite-type-alias-mutual-recursion.stderr

@@ -0,0 +1,34 @@
+error[E0391]: cycle detected when expanding type alias `X1`
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:1:11
+   |
+LL | type X1 = X2;
+   |           ^^
+   |
+note: ...which requires expanding type alias `X2`...
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:3:11
+   |
+LL | type X2 = X3;
+   |           ^^
+note: ...which requires expanding type alias `X3`...
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:4:11
+   |
+LL | type X3 = X1;
+   |           ^^
+   = note: ...which again requires expanding type alias `X1`, completing the cycle
+   = note: type aliases cannot be recursive
+   = help: consider using a struct, enum, or union instead to break the cycle
+   = help: see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information
+note: cycle used when collecting item types in top-level module
+  --> $DIR/infinite-type-alias-mutual-recursion.rs:1:1
+   |
+LL | / type X1 = X2;
+LL | |
+LL | | type X2 = X3;
+LL | | type X3 = X1;
+LL | |
+LL | | fn main() {}
+   | |____________^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.