Encapsulate clang::ASTUnit in SemIR::CppFile. (#6459)

This intends to avoid proliferation of dependencies on the exact API of
`clang::ASTUnit`, and would enable us to more easily switch to a
different approach that gives us more control over the construction of
the Clang AST.

Also remove some unnecessary tracking of the `CppFile` and instead
always retrieve it from the `SemIR::File`.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/sem_ir/cpp_file.cpp

@@ -0,0 +1,19 @@
+// 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/sem_ir/cpp_file.h"
+
+namespace Carbon::SemIR {
+
+auto CppFile::VisitLocalTopLevelDecls(
+    llvm::function_ref<void(const clang::Decl*)> visitor) const -> void {
+  ast_unit_->visitLocalTopLevelDecls(
+      &visitor, [](void* erased_visitor_ptr, const clang::Decl* decl) {
+        auto* visitor_ptr = static_cast<decltype(visitor)*>(erased_visitor_ptr);
+        (*visitor_ptr)(decl);
+        return true;
+      });
+}
+
+}  // namespace Carbon::SemIR