Factor out a Check::CppContext holding C++-specific check state (#6482)

* Move `Sema` access from `CppFile` into `CppContext`.
* Move the mangle context from `SemIR::File` into `CppContext`.
* Move source location mapping state from `Context` into `CppContext`.

Also factor out the `GenerateAst` function that builds the `CppContext`
and `CppFile` into its own file.

toolchain/check/cpp/context.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/context.h"
+
+#include "clang/AST/Mangle.h"
+
+namespace Carbon::Check {
+
+CppContext::CppContext(clang::ASTUnit* ast_unit) : ast_unit_(ast_unit) {}
+
+CppContext::~CppContext() = default;
+
+auto CppContext::clang_mangle_context() -> clang::MangleContext& {
+  if (!clang_mangle_context_) {
+    clang_mangle_context_.reset(ast_context().createMangleContext());
+  }
+  return *clang_mangle_context_;
+}
+
+}  // namespace Carbon::Check