Introduce forward_scope_exit

Introduce forward_scope_exit

This adds a template that can be used to automatically instantiate
scope_exit-like types that wrap some cleanup function.  The
instantiated type has a ctor that has the same interface as the
wrapped function.  While the "magic" is just straight C++11, the
intended use is via the FORWARD_SCOPE_EXIT macro, which is a minimal
macro that avoids spelling out the wrapped function name more than
once:

 void some_function (int foo, object *bar);
 using some_function_fce = FORWARD_SCOPE_EXIT (some_function);
 some_function_fce cleanup (some_int, some_obj_ptr);

The above runs:
  some_function (some_int, some_obj_ptr);
at scope exit.

This is mainly useful as opposed to a simpler SCOPE_EXIT when you need
to:
  - cancel the scope_exit, in which case you need the object's name
  - wrap the scope_exit in a gdb::optional, in which case you need the
    scope_exit's type in advance.

More details in the code comments.

gdb/ChangeLog:
2019-01-23  Pedro Alves  <palves@redhat.com>
	    Andrew Burgess  <andrew.burgess@embecosm.com>

	* common/forward-scope-exit.h: New file.