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/Serialization/SerializedModuleLoader.cpp

@@ -720,7 +720,8 @@ void swift::serialization::diagnoseSerializedASTLoadFailure(
   case serialization::Status::FailedToLoadBridgingHeader:
     // We already emitted a diagnostic about the bridging header. Just emit
     // a generic message here.
-    Ctx.Diags.diagnose(diagLoc, diag::serialization_load_failed, ModuleName);
+    Ctx.Diags.diagnose(diagLoc, diag::serialization_load_failed,
+                       ModuleName.str());
     break;
 
   case serialization::Status::NameMismatch: {