[llvm-c][NFC] fix build warning for reinterpret_cast

[liaolucy]

warning:

llvm-project/llvm/lib/IR/Core.cpp:90:18: warning: cast between incompatible function types from ‘LLVMDiagnosticHandler’ {aka ‘void (*)(LLVMOpaqueDiagnosticInfo*, void*)’} to ‘llvm::DiagnosticHandler::DiagnosticHandlerTy’ {aka ‘void (*)(const llvm::DiagnosticInfo&, void*)’} [-Wcast-function-type]
   90 |           Handler),
      |                  ^

llvm/lib/IR/Core.cpp

 }
 
 LLVMContextRef LLVMGetGlobalContext() { return wrap(&*GlobalContext); }
 
 void LLVMContextSetDiagnosticHandler(LLVMContextRef C,
                                      LLVMDiagnosticHandler Handler,
                                      void *DiagnosticContext) {
   unwrap(C)->setDiagnosticHandlerCallBack(
-      LLVM_EXTENSION reinterpret_cast<DiagnosticHandler::DiagnosticHandlerTy>(
+      LLVM_EXTENSION reinterpret_cast<DiagnosticHandler::DiagnosticHandlerTy &>(
           Handler),
       DiagnosticContext);
 }
 
 LLVMDiagnosticHandler LLVMContextGetDiagnosticHandler(LLVMContextRef C) {
   return LLVM_EXTENSION reinterpret_cast<LLVMDiagnosticHandler>(
       unwrap(C)->getDiagnosticHandlerCallBack());
 }