[Sema] Accept and store the IPI library-level
Prepare to accept the `ipi` argument to the `-library-level` flag. IPI
stands for Internal Programming Interface and would describe a module
that's not to be distributed outside of its project.
In the future, the compiler could use that information to report when a
distributed module (api or spi) imports publicly a module that's not
distributed.
rdar://102435183
