Support for terse => function bodies (#7895)

Treat the function as having a `-> auto` return type. Reject calls to
functions that still have a non-deduced `-> auto` return type, and
rewrite the function to have the actual return type once it is known.

Assisted-by: Gemini via Antigravity

toolchain/parse/handle_function.cpp

@@ -74,7 +74,10 @@ auto HandleFunctionSignatureFinish(Context& context) -> void {
       break;
     }
     case Lex::TokenKind::EqualGreater: {
-      context.AddFunctionDefinitionStart(context.Consume(), state.has_error);
+      context.AddFunctionTerseDefinitionStart(context.Consume(),
+                                              state.has_error);
+      // Any error is recorded on the FunctionTerseDefinitionStart.
+      state.has_error = false;
       context.PushState(state, StateKind::FunctionTerseBodyFinish);
       context.PushStateForExpr(PrecedenceGroup::ForTopLevelExpr());
       break;