Test that the swiftinterface for the stdlib can be compiled (#24128)

...like we've done with the overlays for months. Meant to do this a
long time ago, but at the time we hadn't gotten the stdlib working
yet!

Author: jrose-apple

validation-test/ParseableInterface/verify_stdlib.swift

@@ -0,0 +1,7 @@
+// Note that this test should still "pass" when Swift.swiftinterface has not
+// been generated.
+
+// RUN: %empty-directory(%t)
+// RUN: not ls %platform-module-dir/Swift.swiftmodule/%target-cpu.swiftinterface || %target-swift-frontend -build-module-from-parseable-interface %platform-module-dir/Swift.swiftmodule/%target-cpu.swiftinterface -parse-stdlib -o %t/Swift.swiftmodule
+
+// REQUIRES: nonexecutable_test