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/context.cpp

@@ -6,20 +6,24 @@
 
 #include "clang/AST/Mangle.h"
 #include "clang/Frontend/CompilerInstance.h"
-#include "clang/Parse/Parser.h"
+#include "toolchain/check/cpp/domain.h"
 
 namespace Carbon::Check {
 
-CppContext::CppContext(clang::CompilerInstance& instance,
-                       std::shared_ptr<clang::Parser> parser,
+CppContext::CppContext(CppDomain& domain,
                        std::unique_ptr<CppDiagnosticListener> listener)
-    : ast_context_(&instance.getASTContext()),
-      sema_(&instance.getSema()),
-      parser_(std::move(parser)),
-      diagnostic_listener_(std::move(listener)) {}
+    : domain_(&domain), diagnostic_listener_(std::move(listener)) {}
 
 CppContext::~CppContext() = default;
 
+auto CppContext::ast_context() -> clang::ASTContext& {
+  return domain_->clang_instance().getASTContext();
+}
+
+auto CppContext::sema() -> clang::Sema& {
+  return domain_->clang_instance().getSema();
+}
+
 auto CppContext::clang_mangle_context() -> clang::MangleContext& {
   if (!clang_mangle_context_) {
     clang_mangle_context_.reset(ast_context().createMangleContext());