Support `ConstArgKind::Path`s for const struct/variant constructors

[camelid]

Requires encoding const ctors in MIR metadata (not currently done for some reason). May affect perf. cc #131081.

edit: BoxyUwU

We probably want to lower paths to const ctors directly to a ConstKind::Value to avoid needing to encode the MIR for const ctors. This is technically "lossy" lowering in some sense but it seems fine to me.