Defer parsing of method bodies until the end of a suitable enclosing scope. (#3832)

In parse, form a list of methods that are defined inline, tracking where
they start, where they end, and which other inline methods are nested
within them.

In check, when we reach an inline method body, skip it and add it to a
worklist to be processed later. We also track when we reach the start
and end of a context in which inline method bodies are deferred, so that
we know when to replay the bodies.

When suspending a function definition to be processed later, the
`DeclNameStack` entry is moved to separate storage, including popping
the corresponding scopes from the scope stack and removing the
corresponding lexical names from lexical lookup. Later, when we return
to the function and parse its definition, the `DeclNameStack` entry is
restored. The same is done when we reach the end of a nested context
that can have inline methods, so that we can reenter the nested scope
before processing its members.

---------

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

toolchain/parse/handle_function.cpp

@@ -44,8 +44,8 @@ auto HandleFunctionSignatureFinish(Context& context) -> void {
       break;
     }
     case Lex::TokenKind::OpenCurlyBrace: {
-      context.AddNode(NodeKind::FunctionDefinitionStart, context.Consume(),
-                      state.subtree_start, state.has_error);
+      context.AddFunctionDefinitionStart(context.Consume(), state.subtree_start,
+                                         state.has_error);
       // Any error is recorded on the FunctionDefinitionStart.
       state.has_error = false;
       context.PushState(state, State::FunctionDefinitionFinish);
@@ -93,8 +93,8 @@ auto HandleFunctionSignatureFinish(Context& context) -> void {
 
 auto HandleFunctionDefinitionFinish(Context& context) -> void {
   auto state = context.PopState();
-  context.AddNode(NodeKind::FunctionDefinition, context.Consume(),
-                  state.subtree_start, state.has_error);
+  context.AddFunctionDefinition(context.Consume(), state.subtree_start,
+                                state.has_error);
 }
 
 }  // namespace Carbon::Parse