docs: add a section about formal vs lowered types in the SIL documentation

[eeckstein]

No description provided.

[eeckstein]

@swift-ci smoke test

[atrick]

@eeckstein @slavapestov I've never seen documentation explain the most confusing part to me.
If I have a lowered type, and I walk that type or ask for one of its component types, won't I get back a formal type? So don't I need to explicitly ask for the lowered type every time I do that?
A lowered function type has formal parameter types right!

[eeckstein]

So don't I need to explicitly ask for the lowered type every time I do that?

Yes I think so. For example, that's what's happening in SILType::getFieldType which gets the field types of e.g. a struct.

[meg-gupta]

thanks!

[atrick]

Yes I think so. For example, that's what's happening in SILType::getFieldType

right, maybe it's worth saying something about that. I think that's where we had bugs