gdb: make_scoped_restore and types convertible to T

gdb: make_scoped_restore and types convertible to T

A following patch will want to do

   string_file str_file;

   scoped_restore save_stdout
    = make_scoped_restore (&gdb_stdout, &str_file);

where gdb_stdout is a ui_file *, and string_file is a type that
inherits from ui_file, but that doesn't compile today:

  src/gdb/top.c: In function ‘std::__cxx11::string execute_command_to_string(char*, int)’:
  src/gdb/top.c:710:50: error: no matching function for call to ‘make_scoped_restore(ui_file**, string_file*)’
       = make_scoped_restore (&gdb_stdout, &str_file);
						    ^
  [...]
  In file included from src/gdb/utils.h:25:0,
		   from src/gdb/defs.h:732,
		   from src/gdb/top.c:20:
  src/gdb/common/scoped_restore.h:94:24: note: candidate: template<class T> scoped_restore_tmpl<T> make_scoped_restore(T*, T)
   scoped_restore_tmpl<T> make_scoped_restore (T *var, T value)
			  ^
  src/gdb/common/scoped_restore.h:94:24: note:   template argument deduction/substitution failed:
  src/gdb/top.c:710:50: note:   deduced conflicting types for parameter ‘T’ (‘ui_file*’ and ‘string_file*’)
       = make_scoped_restore (&gdb_stdout, &str_file);
						    ^

This commit makes code such as the above possible.

gdb/ChangeLog:
2017-01-31  Pedro Alves  <palves@redhat.com>

	* common/scoped_restore.h
	(scoped_restore_tmpl::scoped_restore_tmpl): Template on T2, and
	change the value's parameter type to T2.
	(make_scoped_restore): Likewise.