Keep libtool from complaining about ExecutablePath.cpp being empty. (#1402)

On non-OpenBSD (so, everywhere), libtool complains that
ExecutablePath.cpp is empty. Silence it by providing a dummy symbol.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/_TestingInternals/ExecutablePath.cpp

@@ -29,8 +29,12 @@ static void captureEarlyCWD(void) {
     earlyCWD.store(buffer);
   }
 }
+#endif
 
 const char *swt_getEarlyCWD(void) {
+#if defined(__OpenBSD__)
   return earlyCWD.load();
-}
+#else
+  return nullptr;
 #endif
+}

Sources/_TestingInternals/include/ExecutablePath.h

@@ -16,15 +16,15 @@
 
 SWT_ASSUME_NONNULL_BEGIN
 
-#if defined(__OpenBSD__)
 /// Get the current working directory as it was set shortly after the process
 /// started and before `main()` has been called.
 ///
 /// This function is necessary on OpenBSD so that we can (as correctly as
 /// possible) resolve the executable path when the first argument is a relative
 /// path (which can occur when manually invoking the test executable.)
+///
+/// On all other platforms, this function always returns `nullptr`.
 SWT_EXTERN const char *_Nullable swt_getEarlyCWD(void);
-#endif
 
 SWT_ASSUME_NONNULL_END