Rework name lookup to handle non-lexical scoping. (#3354)

When declaring a name such as `fn Ns.Class.F() { ... }`, enter the
scopes of `Ns` and `Ns.Class` as we form the name, and remain in those
non-lexical scopes until the end of the declaration.

When performing an unqualified lookup, look in any enclosing non-lexical
scopes in addition to looking into the lexical name table.

We now track a scope index with each lookup result in the lexical name
lookup table. This is used to determine whether a lexical or non-lexcial
result is the innermost result and whether a declared name is in the
same scope as some previous introduction of that name or in a nested
scope. For now, this could just be the index into the scope_stack, but
the intent is to also use this to detect names being declared after they
are first looked up, which requires the indexes to outlive their scopes,
so we use a persistent numbering of all scopes instead. The persistent
numbering also permits more invariant checking.

toolchain/check/handle_function.cpp

@@ -55,7 +55,7 @@ static auto BuildFunctionDeclaration(Context& context, bool is_definition)
       context.node_stack()
           .PopIf<Parse::NodeKind::ImplicitParameterList>()
           .value_or(SemIR::InstBlockId::Empty);
-  auto name_context = context.declaration_name_stack().Pop();
+  auto name_context = context.declaration_name_stack().FinishName();
   auto fn_node =
       context.node_stack()
           .PopForSoloParseNode<Parse::NodeKind::FunctionIntroducer>();
@@ -132,6 +132,7 @@ static auto BuildFunctionDeclaration(Context& context, bool is_definition)
 auto HandleFunctionDeclaration(Context& context, Parse::Node /*parse_node*/)
     -> bool {
   BuildFunctionDeclaration(context, /*is_definition=*/false);
+  context.declaration_name_stack().PopScope();
   return true;
 }
 
@@ -156,6 +157,7 @@ auto HandleFunctionDefinition(Context& context, Parse::Node parse_node)
   context.PopScope();
   context.inst_block_stack().Pop();
   context.return_scope_stack().pop_back();
+  context.declaration_name_stack().PopScope();
   return true;
 }
 
@@ -233,7 +235,7 @@ auto HandleFunctionIntroducer(Context& context, Parse::Node parse_node)
   // Push the bracketing node.
   context.node_stack().Push(parse_node);
   // A name should always follow.
-  context.declaration_name_stack().Push();
+  context.declaration_name_stack().PushScopeAndStartName();
   return true;
 }