test(ffi-linkage): Phase 8.0 — xfail #foreign-removed rejection diagnostic

Add examples/1176-diagnostics-foreign-removed.sx pinning the DESIRED Phase 8 cutover
behavior: a bare '#foreign' decl must be rejected with a clear migration message
('#foreign has been removed; use the postfix extern/export'). RED — '#foreign' still
parses (routes onto extern) so the decl compiles and exits 0 instead of erroring.
The very next commit (8.1, parser hard-reject) greens it.

examples/expected/1176-diagnostics-foreign-removed.exit

@@ -0,0 +1 @@
+1

examples/expected/1176-diagnostics-foreign-removed.stderr

@@ -0,0 +1,5 @@
+error: `#foreign` has been removed; use the postfix `extern` (import) / `export` (define) linkage keyword instead
+  --> examples/1176-diagnostics-foreign-removed.sx:8:26
+   |
+ 8 | abs_c :: (n: i32) -> i32 #foreign;
+   |                          ^^^^^^^^