Lowering support for overloaded operators. (#3798)

Fix a collection of issues that were preventing lowering for overloaded
operators from working.

Instead of creating `import_ref` instructions during name lookup in the
current block, whatever that might be, we now create them in the `file`
block always. This avoids inserting them into blocks that might not be
intended to contain them, such as functions, and avoids the IR generated
for a function depending on which names we happen to have looked up
first.

When importing a class, function, or interface, import its enclosing
scope ID. This is necessary to allow us to distinguish between functions
at interface scope, which shouldn't be lowered, and other functions, and
will also be used in future to provide qualified names for declarations
when printing types. In order to support this:

- Track the constant values of namespaces created during importing so
that we can find them when resolving an import ref. Use those constant
values to convert an enclosing scope ID from the imported IR into a
corresponding ID in the current IR.
- Change how we do two-pass import of classes and namespaces so that we
can do two-pass import even for non-defining declarations, so that we
can import the enclosing scope.

While working on the final point above, I reworked `TryResolveInst` to
return a flag indicating whether another pass is necessary instead of
implicitly encoding this in the `ConstantId`. This permits the handling
of classes to be simplified; now `import_ir_constant_values` is only
accessed in a single place.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/check.cpp

@@ -159,6 +159,15 @@ struct UnitInfo {
 // Add imports to the root block.
 static auto InitPackageScopeAndImports(Context& context, UnitInfo& unit_info)
     -> void {
+  // First create the constant values map for all imported IRs. We'll populate
+  // these with mappings for namespaces as we go.
+  size_t num_irs = context.import_irs().size();
+  for (auto& [_, package_imports] : unit_info.package_imports_map) {
+    num_irs += package_imports.imports.size();
+  }
+  context.import_ir_constant_values().resize(
+      num_irs, SemIR::ConstantValueStore(SemIR::ConstantId::Invalid));
+
   // Importing makes many namespaces, so only canonicalize the type once.
   auto namespace_type_id =
       context.GetBuiltinType(SemIR::BuiltinKind::NamespaceType);
@@ -217,9 +226,8 @@ static auto InitPackageScopeAndImports(Context& context, UnitInfo& unit_info)
                                     sem_irs, package_imports.has_load_error);
   }
 
-  context.import_ir_constant_values().resize(
-      context.import_irs().size(),
-      SemIR::ConstantValueStore(SemIR::ConstantId::Invalid));
+  CARBON_CHECK(context.import_irs().size() == num_irs)
+      << "Created an unexpected number of IRs";
 }
 
 // Loops over all nodes in the tree. On some errors, this may return early,