Enable CMake policy CMP0057, which allows `if()` statements to use the `IN_LIST`
operator. In addition, simplify several `if()` statements that used the
`list(FIND ...)` operation instead.
Introduce two new aliasees `ALL_POSIX_PLATFORMS` and `ALL_APPLE_PLATFORMS`. The
latter expands to iOS, tvOS, watchOS, macOS and if relevant, the simulators.
The former expands to the same list as `ALL_APPLE_PLATFORMS` as well as Linux
and FreeBSD.
swift-reflection-dump, a host-side tool, requires this library that
normally builds for the configured platform. So, if the host platform
isn't configured to build for some reason, swift-reflection-dump will
fail to link.