Add some encapsulation to CppDomain (#7579)

Start tracking the domain within `CppContext`s instead of having them
duplicate its fields. This allows us to remove the shared ownership of
the clang parser.

---------

Co-authored-by: Christopher Di Bella <cjdb.ns@gmail.com>

toolchain/check/cpp/domain.cpp

@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/cpp/domain.h"
+
+#include "clang/Parse/Parser.h"
+
+namespace Carbon::Check {
+
+CppDomain::CppDomain(std::shared_ptr<clang::CompilerInstance> clang_instance,
+                     std::unique_ptr<clang::Parser> parser,
+                     clang::CodeGenerator* code_generator,
+                     llvm::LLVMContext* llvm_context)
+    : clang_instance_(std::move(clang_instance)),
+      parser_(std::move(parser)),
+      code_generator_(code_generator),
+      llvm_context_(llvm_context) {}
+
+CppDomain::~CppDomain() = default;
+
+}  // namespace Carbon::Check