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/language_server/context.cpp

@@ -148,14 +148,12 @@ auto Context::File::SetText(Context& context, std::optional<int64_t> version,
 
   SemIR::File sem_ir(tree_.get(), SemIR::CheckIRId(0), tree_->packaging_decl(),
                      *value_stores_, uri_.file().str());
-  std::unique_ptr<clang::ASTUnit> clang_ast_unt;
   // TODO: Support cross-file checking when multiple files have edits.
   llvm::SmallVector<Check::Unit> units = {{{.consumer = &consumer,
                                             .value_stores = value_stores_.get(),
                                             .timings = nullptr,
                                             .sem_ir = &sem_ir,
-                                            .total_ir_count = 1,
-                                            .clang_ast_unit = &clang_ast_unt}}};
+                                            .total_ir_count = 1}}};
 
   auto getter = [this]() -> const Parse::TreeAndSubtrees& {
     return *tree_and_subtrees_;