Reduce WOW64 code duplication

Reduce WOW64 code duplication

Currently we have duplicate code for each place where
windows_thread_info::context is touched, since for WOW64 processes
it has to do the equivalent with wow64_context instead.

For example this code...:

 #ifdef __x86_64__
   if (windows_process.wow64_process)
     {
       th->wow64_context.ContextFlags = WOW64_CONTEXT_ALL;
       CHECK (Wow64GetThreadContext (th->h, &th->wow64_context));
       ...
     }
   else
 #endif
     {
       th->context.ContextFlags = CONTEXT_DEBUGGER_DR;
       CHECK (GetThreadContext (th->h, &th->context));
       ...
     }

...changes to look like this instead:

   windows_process.with_context (th, [&] (auto *context)
     {
       context->ContextFlags = WindowsContext<decltype(context)>::all;
       CHECK (get_thread_context (th->h, context));
       ...
     }

The actual choice if context or wow64_context are used, is handled by
this new function in windows_process_info:

   template<typename Function>
   auto with_context (windows_thread_info *th, Function function)
   {
 #ifdef __x86_64__
     if (wow64_process)
       return function (th != nullptr ? th->wow64_context : nullptr);
     else
 #endif
       return function (th != nullptr ? th->context : nullptr);
   }

The other parts to make this work are the templated WindowsContext class
which give the appropriate ContextFlags for both types.
And there are also overloaded helper functions, like in the case of
get_thread_context here, call either GetThreadContext or
Wow64GetThreadContext.

According git log --stat, this results in 120 lines less code.

Approved-By: Tom Tromey <tom@tromey.com>