Import support for generics and specifics (#4179)

Import generics and specifics when they are referenced by imported
entities.

When importing a generic, we import the symbolic constants required by
its eval block, and then rebuild the eval block itself given the list of
constants it needs to compute. This is likely a bit less efficient than
directly importing the contents of the eval block, but avoids needing to
either extend the importer code to be able to import the instructions
that can appear in the eval block or extend the evaluator to cope with
instructions from a different `SemIR::File`.

Importing a symbolic constant is unaffected, and does not yet preserve
the associated generic and index within that generic, so uses of a
generic from an imported IR still don't pick up values from the
specific, but the improved functionality can be seen in the changes to
the SemIR in the testcases.

toolchain/sem_ir/generic.cpp

@@ -32,6 +32,7 @@ class SpecificStore::KeyContext : public TranslatingKeyContext<KeyContext> {
 
 auto SpecificStore::GetOrAdd(GenericId generic_id, InstBlockId args_id)
     -> SpecificId {
+  CARBON_CHECK(generic_id.is_valid());
   return lookup_table_
       .Insert(
           KeyContext::Key{.generic_id = generic_id, .args_id = args_id},