fix: use std::string for JIT entrypoint funcname

As we store the funcname for the entrypoint for later use, a pointer
might point to some random memory when used as there is no special
management for that name at the higher levels.

compiler/include/zamalang/Support/Jit.h

@@ -104,7 +104,7 @@ public:
 
 private:
   mlir::LLVM::LLVMFunctionType type;
-  llvm::StringRef name;
+  std::string name;
   std::unique_ptr<mlir::ExecutionEngine> engine;
 };