Merge #20495: sync: Use decltype(auto) return type for WITH_LOCK

3eb94ec81b72b14f72a1f6ce5c9aa24476df755a sync: Use decltype(auto) return type for WITH_LOCK (Carl Dong)

Pull request description:

  > Now that we're using C++17, we can use the decltype(auto) return type
  > for functions and lambda expressions.
  >
  > As demonstrated in this commit, this can simplify cases where previously
  > the compiler failed to deduce the correct return type.
  >
  > Just for reference, for the "assign to ref" cases fixed here, there are
  > 3 possible solutions:
  >
  > - Return a pointer and immediately deref as used before this commit
  > - Make sure the function/lambda returns declspec(auto) as used after
  >   this commit
  > - Class& i = WITH_LOCK(..., return std::ref(...));
  >
  > -----
  >
  > References:
  > 1. https://en.cppreference.com/w/cpp/language/function#Return_type_deduction
  > 2. https://en.cppreference.com/w/cpp/language/template_argument_deduction#Other_contexts
  > 3. https://en.cppreference.com/w/cpp/language/auto
  > 4. https://en.cppreference.com/w/cpp/language/decltype
  >
  > Explanations:
  > 1. https://stackoverflow.com/a/21369192
  > 2. https://stackoverflow.com/a/21369170

  Thanks to sipa and ryanofsky for helping me understand this

ACKs for top commit:
  jnewbery:
    utACK 3eb94ec81b72b14f72a1f6ce5c9aa24476df755a
  hebasto:
    ACK 3eb94ec81b72b14f72a1f6ce5c9aa24476df755a, I have reviewed the code and it looks OK, I agree it can be merged. I have verified possible warnings:
  ryanofsky:
    Code review ACK 3eb94ec81b72b14f72a1f6ce5c9aa24476df755a

Tree-SHA512: 5f55c7722aeca8ea70e5c1a8db93e93ba0e356e8967e7f607ada38003df4b153d73c29bd2cea8d7ec1344720d37d857ea7dbfd2a88da1d92e0e9cbb9abd287df

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