Zcu: convert Decl.zir_inst_index to a TrackedInst.Index.Optional
This makes tracking easier across incremental updates: `scanDecl` can
now tell whether an existing decl in a namespace was mapped to the one
it's analyzing in the new ZIR.
d4ce1ec8ddmlugg committed on 3/14/2024, 7:40:06 AMparent7c4793f