Lazily lower entities rather than iterating over all entities in SemIR

[dwblaikie]

Description of the bug:

We currently emit function forward declarations for totally unused/unreferenced function forward declarations.
This is unnecessary & we should avoid doing it, by having function and other entities emitted lazily - starting at the definitions, emitting declarations for anything those definitions call. Some of this is already implemented for emitting specialized generics and could be generalized further to handle the non-generic cases lazily as well.

(this was originally discovered with a crash on code like this: class C; fn F(c: C); where C has not been completed, has no value representation and lowering crashes expecting there to be one - this case may be fixed in other ways, though)

Discussed on discord starting here: https://discord.com/channels/655572317891461132/655578254970716160/1344750580840529982