[CodeGen] Fix -Wmismatched-tags in StackProtector.h (NFC)

llvm-project/llvm/include/llvm/CodeGen/StackProtector.h:69:10:
error: class 'AnalysisInfoMixin' was previously declared as a struct; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Werror,-Wmismatched-tags]
   69 |   friend class AnalysisInfoMixin<SSPLayoutAnalysis>;
      |          ^
llvm-project/llvm/include/llvm/IR/PassManager.h:414:8: note: previous use is here
  414 | struct AnalysisInfoMixin : PassInfoMixin<DerivedT> {
      |        ^
llvm-project/llvm/include/llvm/CodeGen/StackProtector.h:69:10: note: did you mean struct here?
   69 |   friend class AnalysisInfoMixin<SSPLayoutAnalysis>;
      |          ^~~~~
      |          struct
1 error generated.