gccrs: Add regression test for invalid label name

An error message should be emitted when the rust code contains invalid
label name. Add a new test for this behavior.

gcc/testsuite/ChangeLog:

	* rust/compile/invalid_label_name.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

gcc/testsuite/rust/compile/invalid_label_name.rs

+pub fn function() {
+    'continue: loop {
+        // { dg-error "invalid label name .'continue." "" { target *-*-* } .-1 }
+        break 'extern;
+        // { dg-error "invalid label name .'extern." "" { target *-*-* } .-1 }
+    }
+
+    'break: loop {
+        // { dg-error "invalid label name .'break." "" { target *-*-* } .-1 }
+        break 'for;
+        // { dg-error "invalid label name .'for." "" { target *-*-* } .-1 }
+    }
+
+    'crate: loop {
+        // { dg-error "invalid label name .'crate." "" { target *-*-* } .-1 }
+        break 'loop;
+        // { dg-error "invalid label name .'loop." "" { target *-*-* } .-1 }
+    }
+
+    'a: loop {
+        break 'a;
+    }
+}