The `add_swift_target_executable` is used in a single case, where it is
already being built in a standard library only build. This removes the
unnecessary condition in the build path.
This was used for the swift-reflection-test tool. Instead of using fat
binaries, use the target binary itself. This simplifies the build logic
as well as paves the road to sub-builds for each target rather than a
monolithic build as we have today.
Originally, the swift-reflection-test was a host tool that linked
against the target libraries since it was testing the target layout.
Now that the tool has been split into a host and target component
(5ea5bb06a3) and we have target and host
libraries that we can link against appropriately, we do not need to link
against the FAT binary. Since there was exactly one use of this
functionality, switching that from fat linking to regular linking allows
us to remove this functionality entirely. Switch to regular linking and
remove the option.
This will ensure that additional target executables can not be added to the rest
of the swift project without anyone noticing since the non-stdlib parts of
Swift's cmake will not have visibility of the declaration unless they change the
cmake lookup paths.
