Properly implement `builtin#offset_of`

[Veykril]

We already have the rough parts implemented, but our parse seems to be wrong and the IDE layer needs some help figuring out the syntax as stuff like std::mem::offset_of!(Option<&u8>, Some.0);/builtin#offset_of(Option<&u8>, Some.0); currently errors with parse errors and Some being unresolved.

The macro for this stabilizes in 3 days so

[DropDemBits]

I believe only single-field offset_of is being stabilized and not nested fields (rust-lang/rust#120140
) nor for enum variants (rust-lang/rust#120141), so I don't think we need to worry as much for those cases.

I don't know about the parsing & resolution situation for the single-field offset_of, but I'm hoping it's in a better place than the other two?

[Veykril]

The field in builtin #offset_of(Struct, field) also doesn't resolve on the IDE layer since we need to special case the handling for such a syntax node.