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/driver/compile_subcommand.cpp

@@ -571,7 +571,6 @@ class CompilationUnit {
   std::optional<std::function<auto()->const Parse::TreeAndSubtrees&>>
       tree_and_subtrees_getter_;
   std::optional<SemIR::File> sem_ir_;
-  std::unique_ptr<clang::ASTUnit> clang_ast_unit_;
   std::unique_ptr<llvm::LLVMContext> llvm_context_;
   std::unique_ptr<llvm::Module> module_;
   std::unique_ptr<llvm::TargetMachine> target_machine_;
@@ -764,8 +763,7 @@ auto CompilationUnit::GetCheckUnit() -> Check::Unit {
           .value_stores = &value_stores_,
           .timings = timings_ ? &*timings_ : nullptr,
           .sem_ir = &*sem_ir_,
-          .total_ir_count = total_ir_count_,
-          .clang_ast_unit = &clang_ast_unit_};
+          .total_ir_count = total_ir_count_};
 }
 
 auto CompilationUnit::PostCheck() -> void {