Fix clang/libc++ build

Fix clang/libc++ build

This fixes the following build error with clang/libc++, reported at
<https://sourceware.org/ml/gdb-patches/2019-01/msg00537.html>:

  (...)
  In file included from breakpoint.c:34:
  In file included from ./inferior.h:54:
  ./common/forward-scope-exit.h:98:7: error: no matching constructor for
  initialization of 'decltype(std::bind(&delete_longjmp_breakpoint,
  std::declval<int>()))' (aka '__bind<void (*)(int), int>')
      : m_bind_function (std::bind (function, args...))
	^                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ./common/gdb_optional.h:155:19: note: in instantiation of member
  function 'detail::forward_scope_exit<void (int),
  &delete_longjmp_breakpoint, void (int)>::forward_scope_exit' requested
  here
      new (&m_item) T (std::forward<Args>(args)...);
		    ^
  breakpoint.c:11127:18: note: in instantiation of function template
  specialization 'gdb::optional<detail::forward_scope_exit<void (int),
  &delete_longjmp_breakpoint, void (int)> >::emplace<int &>' requested
  here
	lj_deleter.emplace (thread);
		   ^
  /Applications/Xcode-10.1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/functional:2220:7:
  note: candidate constructor (the implicit copy constructor) not
  viable: no known conversion from '__bind<[...], int &>' to 'const
  __bind<[...], int>' for 1st argument
  class __bind
	^
  (...)

I don't really know why I ended up with a copy here.  We can just pass
the arguments directly to the being-constructed bind.

gdb/ChangeLog:
2019-01-24  Pedro Alves  <palves@redhat.com>

	* common/forward-scope-exit.h
	(forward_scope_exit::forward_scope_exit): Pass arguments to
	m_bind_function directly, instead of creating a std::bind and
	copying that.