[clang] [CodeGen] Apply 'readonly' to 'this' pointer argument. (PR #106499)

[Zhijin Zeng via cfe-commits]

zengdage wrote:

@nikic @ChuanqiXu9 I am sorry to bother you again. Do you think the attribute 'nofree' should be added to 'this' pointer argument  in c++ instance method ?  I think the 'this' pointer pointed to current object and it's life time is guaranteed to be longer than the instance method's lifetime.  If can, we can add `hasNoFreeAttr` check for 'Argument' case in `Value::canBeFreed`.

https://github.com/llvm/llvm-project/pull/106499