Add lattice element to represent valid tparams
Currently we represent an unknown :static_parameter, as `Any`.
However, we actually have a fair bit of additional information
about it. In particular, a value that came from a static parameter
must have been a `valid_tparam` and thus if it's used in `apply_type`
again, we're guaranteed that such a use will be nothrow. This adds
a special lattice element to encode this set of values. The primary
benefit is to be able to prove the nothrow fact about many more
apply_type calls and thus cut down the size of the IR. That said,
at the moment, we cannot prove nothrow for unknown `:static_parameter`
either, but that's a separate bit of work (and we need to do both
to really be able to eliminate common patterns).
