Note [Core top-level string literals]

As an exception to the usual rule that top-level binders must be lifted,
we allow binding primitive string literals (of type Addr#) at the
top level. This allows us to share string literals earlier in the pipeline and
crucially allows other optimizations in the Core2Core pipeline to fire.
Consider,

  f n = let a::Addr# = "foo"#
        in \x -> blah

In order to be able to inline `f`, we would like to float `a` to the top.
Another option would be to inline `a`, but that would lead to duplicating string
literals, which we want to avoid. See #8472.

The solution is simply to allow top-level unlifted binders. We can't allow
arbitrary unlifted expression at the top-level though, unlifted binders cannot
be thunks, so we just allow string literals.

We allow the top-level primitive string literals to be wrapped in Ticks
in the same way they can be wrapped when nested in an expression.
CoreToSTG currently discards Ticks around top-level primitive string literals.
See #14779.

Also see Note [Compilation plan for top-level string literals].