JSSpecCompiler: Add converter from LibCpp's AST

This will effectively allow us to use C++ code as an input for the
compiler. This would be useful for testing, since otherwise we would
have had to specify tests as a spec-like XML, which is not exactly the
most developer-friendly experience.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/CppASTConverter.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibCpp/AST.h>
+
+#include "Forward.h"
+
+namespace JSSpecCompiler {
+
+class CppASTConverter {
+public:
+    CppASTConverter(RefPtr<Cpp::FunctionDeclaration> const& function)
+        : m_function(function)
+    {
+    }
+
+    NonnullRefPtr<FunctionDefinition> convert();
+
+private:
+    template<typename T>
+    NullableTree convert_node(T const&);
+    NullableTree as_nullable_tree(Cpp::Statement const* statement);
+    Tree as_tree(Cpp::Statement const* statement);
+    Tree as_possibly_empty_tree(Cpp::Statement const* statement);
+
+    RefPtr<Cpp::FunctionDeclaration> m_function;
+};
+
+}