[Serialization] Add warning when .swiftsourceinfo is malformed

Output a warning rather than ignoring a malformed .swiftsourceinfo
completely.

Resolves rdar://77350048

lib/Serialization/SerializedModuleLoader.cpp

@@ -686,10 +686,16 @@ LoadedFile *SerializedModuleLoaderBase::loadAST(
     std::unique_ptr<llvm::MemoryBuffer> moduleSourceInfoInputBuffer,
     bool isFramework) {
   assert(moduleInputBuffer);
+
+  // The buffers are moved into the shared core, so grab their IDs now in case
+  // they're needed for diagnostics later.
   StringRef moduleBufferID = moduleInputBuffer->getBufferIdentifier();
   StringRef moduleDocBufferID;
   if (moduleDocInputBuffer)
     moduleDocBufferID = moduleDocInputBuffer->getBufferIdentifier();
+  StringRef moduleSourceInfoID;
+  if (moduleSourceInfoInputBuffer)
+    moduleSourceInfoID = moduleSourceInfoInputBuffer->getBufferIdentifier();
 
   if (moduleInputBuffer->getBufferSize() % 4 != 0) {
     if (diagLoc)
@@ -742,6 +748,12 @@ LoadedFile *SerializedModuleLoaderBase::loadAST(
         (Ctx.LangOpts.AllowModuleWithCompilerErrors &&
          (loadInfo.status == serialization::Status::TargetTooNew ||
           loadInfo.status == serialization::Status::TargetIncompatible))) {
+      if (loadedModuleFile->hasSourceInfoFile() &&
+          !loadedModuleFile->hasSourceInfo())
+        Ctx.Diags.diagnose(diagLocOrInvalid,
+                           diag::serialization_malformed_sourceinfo,
+                           moduleSourceInfoID);
+
       Ctx.bumpGeneration();
       LoadedModuleFiles.emplace_back(std::move(loadedModuleFile),
                                      Ctx.getCurrentGeneration());