Make sure all failures to load module interfaces are diagnosed (#25636)

...specifically, diagnosed in the parent DiagnosticEngine. This not
only provides a better user experience, but makes sure that the
compiler exits with a nonzero exit code even if the module goes
unused.

rdar://problem/50789839

lib/Frontend/ParseableInterfaceModuleLoader.cpp

@@ -18,6 +18,7 @@
 #include "swift/AST/FileSystem.h"
 #include "swift/AST/Module.h"
 #include "swift/AST/ProtocolConformance.h"
+#include "swift/Basic/Defer.h"
 #include "swift/Basic/Lazy.h"
 #include "swift/Basic/Platform.h"
 #include "swift/Basic/STLExtras.h"
@@ -564,6 +565,17 @@ public:
 
       SubInstance.createDependencyTracker(FEOpts.TrackSystemDeps);
 
+      SWIFT_DEFER {
+        // Make sure to emit a generic top-level error if a module fails to
+        // load. This is not only good for users; it also makes sure that we've
+        // emitted an error in the parent diagnostic engine, which is what
+        // determines whether the process exits with a proper failure status.
+        if (SubInstance.getASTContext().hadError()) {
+          diags.diagnose(diagnosticLoc, diag::serialization_load_failed,
+                         moduleName);
+        }
+      };
+
       if (SubInstance.setup(subInvocation)) {
         SubError = true;
         return;