Enable compilation under C++20 with clang++ and its standard library

[eddelbuettel]

When using C++20 with clang++ (version 15) and also adding -stdlib=libc++ to enable use of its standard library,
the removal of std::result_of bites us and compilation fails. This was deprecated in C++17, and removed in C++20. Conditional use of the replacement std::invoke_result works.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Prefereably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Enchufa2]

LGTM!