Use specific operations for setting the compile flags, link flags,
linked libraries, and library search paths. This allows us to use CMake
more effectively, simplifies the logic, and will ensure that flags are
not duplicated.
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.
