Only store Mach header addresses for images containing Swift data. (#351)

We currently try to do minimal work in our `objc_addLoadImageFunc()`
callback on Darwin—we acquire a lock, add each reported Mach header to a
list, relinquish the lock, and immediately return. We then look for
appropriate section data on demand in `enumerateTypeMetadataSections()`.
This is a problem if an image is unloaded after the callback to
`objc_addLoadImageFunc()` returns but before we enumerate images. Images
containing Swift do not support unloading, but pure C images might find
themselves in this situation.

This PR checks for the presence of a `"__swift5_types"` section before
adding an image to the list, so references to images that could be
unloaded won't be held indefinitely.

Resolves rdar://124426864.

### 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/Discovery.cpp

@@ -233,9 +233,18 @@ static SWTMachHeaderList getMachHeaders(void) {
     machHeaders->reserve(_dyld_image_count());
 
     objc_addLoadImageFunc([] (const mach_header *mh) {
-      os_unfair_lock_lock(&lock); {
-        machHeaders->push_back(reinterpret_cast<SWTMachHeaderList::value_type>(mh));
-      } os_unfair_lock_unlock(&lock);
+      auto mhn = reinterpret_cast<SWTMachHeaderList::value_type>(mh);
+
+      // Only store the mach header address if the image contains Swift data.
+      // Swift does not support unloading images, but images that do not contain
+      // Swift code may be unloaded at runtime and later crash
+      // the testing library when it calls enumerateTypeMetadataSections().
+      unsigned long size = 0;
+      if (getsectiondata(mhn, SEG_TEXT, "__swift5_types", &size)) {
+        os_unfair_lock_lock(&lock); {
+          machHeaders->push_back(mhn);
+        } os_unfair_lock_unlock(&lock);
+      }
     });
   });