`std::result_of_t` has been deprecated and replaced with
`std::invoke_result_t`. Update to the newer spelling to avoid the C++17
deprecation warnings when building with a new STL.
They're useful functions to have; I don't think we want them to be API, but
having them as SPI is conceivably useful for other purposes, and avoids everyone
rolling their own copy.
rdar://103397975
`SWIFT_RUNTIME_STDLIB_INTERNAL` does `extern "C"`, so we can't put these
in a namespace and have to use a C-style prefix instead.
Also slightly rearrange the code in `CommandLine.cpp`.
rdar://103397975
Instead of triggering a fatal error on failure, return `nullptr` and
let the caller decide what to do about it. The CommandLine code should
trigger a fatal error, of course.
rdar://103397975
In various places we need to call the Windows API, and because Swift uses UTF-8
for its string representation, we can’t call the ANSI API functions (because the
code page used for the ANSI functions varies depending on the system locale
setting). Instead, we need to use the wide character APIs.
This means that we need to convert UTF-8 to wide character and vice-versa in
various places in the runtime.
rdar://103397975
