Use context.x() instead of context.sema_ir().x() in check/cpp/ when possible (#6109)

Avoid using `const Context&`. We always work with a cmutable `Context&`
(https://github.com/carbon-language/carbon-lang/pull/6094#discussion_r2359199247).

toolchain/check/cpp/thunk.cpp

@@ -517,8 +517,7 @@ static auto BuildThunkBody(clang::Sema& sema,
 auto BuildCppThunk(Context& context, const SemIR::Function& callee_function)
     -> clang::FunctionDecl* {
   clang::FunctionDecl* callee_function_decl =
-      context.sem_ir()
-          .clang_decls()
+      context.clang_decls()
           .Get(callee_function.clang_decl_id)
           .decl->getAsFunction();
   CARBON_CHECK(callee_function_decl);