`std::result_of` was removed in C++20, though the actual removal from
the STL library implementations seems to depend on the version. For
example using xcode 14.2 one gets away with a deprecation warning, but
xcode 14.3 will fail.
As Swift 5.8.1 is still compiled with C++14, we cannot replace
`std::result_of` with `std::invoke_result` in the prebuilding patches
just yet, but we can do that for the extractor itself, patching the
prebuilt package.
An interesting byproduct was finding a problematic `assert` in the
Swift headers. An incomplete `FallthroughStmt` was asserting on having
a destination. I did not find any other sensible way of getting rid of
the crash when running in debug mode than to patch the header.
