Fix lowering of a conversion from a type with a pointer value representation to a type with a copy value representation. (#4467)

We previously generated a `value_bind` instruction of the wrong type,
resulting in lowering building bad LLVM IR.

Also fix another issue exposed by this change, where we would import
constants without marking their types as complete, and then crash in
lowering while trying to lower them. This happens in particular for the
`FunctionType`s of functions in `ImplDecl`s. Address this by skipping
lowering for constants with incomplete types.

toolchain/lower/constant.cpp

@@ -260,6 +260,12 @@ auto LowerConstants(FileContext& file_context,
     }
 
     auto inst = file_context.sem_ir().insts().Get(inst_id);
+    if (inst.type_id().is_valid() &&
+        !file_context.sem_ir().types().IsComplete(inst.type_id())) {
+      // If a constant doesn't have a complete type, that means we imported it
+      // but didn't actually use it.
+      continue;
+    }
     llvm::Constant* value = nullptr;
     CARBON_KIND_SWITCH(inst) {
 #define CARBON_SEM_IR_INST_KIND(Name)            \