[TextAPI] Add an assertion for adding duplicate libraries into a single TBD file (llvm#101744)

This would lead to a malformed TBD file.

Author: cyndyishida

llvm/lib/TextAPI/InterfaceFile.cpp

@@ -87,6 +87,9 @@ void InterfaceFile::addDocument(std::shared_ptr<InterfaceFile> &&Document) {
                                   const std::shared_ptr<InterfaceFile> &RHS) {
                                  return LHS->InstallName < RHS->InstallName;
                                });
+  assert((Pos == Documents.end() ||
+          (*Pos)->InstallName != Document->InstallName) &&
+         "Unexpected duplicate document added");
   Document->Parent = this;
   Documents.insert(Pos, Document);
 }