Commit authored by Committed by Dzmitry Malyshau
Simplify interpolation defaulting.
Replace `Module::apply_common_default_interpolation` with a simpler function that handles a single `Binding` at a time. In exchange for the simplicity, the function must be called at each point function arguments, function results, and struct members are prepared. (Any missed spots will be caught by the verifier.) This approach no longer requires mutating types in the arena, a prerequisite for properly handling type identity. Applying defaults to struct members when the struct declaration is parsed does have a disadvantage, compared to the old whole-module pass: at struct parse time, we don't yet know which pipeline stages the struct will be used in. The best we can do is apply defaults to anything with a `Location` binding. This causes needless qualifiers to appear in some output. However, it seems that our back end languages all tolerate such qualifiers.