The main action here is to sink the creation of the installation rule for all of
the swift host tools into this API. In a latter commit, I will use this API to
create include and build rules for add_swift_host_tool.
This is a flexible way to assert that a binary operation succeeds. For
instance to precondition equality:
precondition(EQUAL FOO BAR)
or string equality:
precondition(STREQUALS FOO BAR)
Since cmake is so loosely typed, it is really important that we
precondition as much as possible to ensure sane behavior.
The contents of that .cmake file are cmake utility functions. Move them into
SwiftUtils.cmake, a file that contains cmake utility functions. This is just a
quick cleanup.
