Switch from clang::ASTUnit to clang::CompilerInstance. (#6483)

This gives us a lot more control over how the compiler is built and
invoked. But no functionality changes are intended in this PR.

toolchain/sem_ir/cpp_file.cpp

@@ -1,19 +0,0 @@
-// 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