Add ConstThreadState to mach_extensions.h and use it everywhere

util/mach/mach_extensions.h

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 29 matching lines @@
 // exception_type_t, it’s reasonable to assume that there cannot be any
 // officially-defined exception_type_t values higher than 31.
 // kMachExceptionSimulated uses a value well outside this range because it does
 // not require a corresponding mask value. Simulated exceptions are delivered to
 // the exception handler registered for EXC_CRASH exceptions using
 // EXC_MASK_CRASH.
 
 //! \brief An exception type to use for simulated exceptions.
 const exception_type_t kMachExceptionSimulated = 'CPsx';
 
+//! \brief A const version of `thread_state_t`.
+//!
+//! This is useful as the \a old_state parameter to exception handler functions.
+//! Normally, these parameters are of type `thread_state_t`, but this allows
+//! modification of the state, which is conceptually `const`.
+using ConstThreadState = const natural_t*;
+
 //! \brief Like `mach_thread_self()`, but without the obligation to release the
 //!     send right.
 //!
 //! `mach_thread_self()` returns a send right to the current thread port,
 //! incrementing its reference count. This burdens the caller with maintaining
 //! this send right, and calling `mach_port_deallocate()` when it is no longer
 //! needed. This is burdensome, and is at odds with the normal operation of
 //! `mach_task_self()`, which does not increment the task port’s reference count
 //! whose result must not be deallocated.
 //!
@@ skipping 28 matching lines @@
 //! appropriate for the system at run time.
 //!
 //! \note `EXC_MASK_ALL` does not include the value of `EXC_MASK_CRASH`.
 //!     Consumers that want `EXC_MASK_ALL` along with `EXC_MASK_CRASH` must use
 //!     ExcMaskAll() | `EXC_MASK_CRASH` explicitly.
 exception_mask_t ExcMaskAll();
 
 }  // namespace crashpad
 
 #endif  // CRASHPAD_UTIL_MACH_MACH_EXTENSIONS_H_