Meta: Generate a helper constructor for single-value IPC responses

When an IPC message returns a single value, we generate a class with a
constructor that is something like:

    class MessageResponse {
        MessageResponse(SingleReturnType value)
            : m_value(move(value))
        {
        }
    };

If that IPC message wants to return a value that SingleReturnType is
constructible from, you have to wrap that return call with braces:

    return { value_that_could_construct_single_return_type };

That isn't really an issue except for when we want to mix TRY semantics
with the return type. If SingleReturnType is constructible from an Error
type (i.e. something similar to ErrorOr), the following doesn't work:

    TRY(fallible_function());

Because MessageResponse would not be constructible from Error. Instead,
we must do some workaround with a custom TRY macro, as in 31bb792.

This patch generates a constructor that makes TRY usable as-is without
any custom macros. We perform a very similar trick in ThrowCompletionOr
inside LibJS. This constructor will allow you to create MessageResponse
from any type that SingleReturnType is constructible from.

Meta/Lagom/Tools/CodeGenerators/IPCCompiler/main.cpp

@@ -303,7 +303,23 @@ public:)~~~");
     @message.pascal_name@(@message.pascal_name@ const&) = default;
     @message.pascal_name@(@message.pascal_name@&&) = default;
     @message.pascal_name@& operator=(@message.pascal_name@ const&) = default;
-    @message.constructor@
+    @message.constructor@)~~~");
+
+    if (parameters.size() == 1) {
+        auto const& parameter = parameters[0];
+        message_generator.set("parameter.type"sv, parameter.type);
+        message_generator.set("parameter.name"sv, parameter.name);
+
+        message_generator.appendln(R"~~~(
+    template <typename WrappedReturnType>
+    requires(!SameAs<WrappedReturnType, @parameter.type@>)
+    @message.pascal_name@(WrappedReturnType&& value)
+        : m_@parameter.name@(forward<WrappedReturnType>(value))
+    {
+    })~~~");
+    }
+
+    message_generator.appendln(R"~~~(
     virtual ~@message.pascal_name@() override {}
 
     virtual u32 endpoint_magic() const override { return @endpoint.magic@; }