Poison non-POD memset & non-trivially-copyable memcpy/memmove

Poison non-POD memset & non-trivially-copyable memcpy/memmove

This patch catches invalid initialization of non-POD types with
memset, at compile time.

This is what I used to catch the problems fixed by the previous
patches in the series:

  $ make -k 2>&1 | grep "deleted function"
  src/gdb/breakpoint.c:951:53: error: use of deleted function ‘void* memset(T*, int, size_t) [with T = bp_location; <template-parameter-1-2> = void; size_t = long unsigned int]’
  src/gdb/breakpoint.c:7325:32: error: use of deleted function ‘void* memset(T*, int, size_t) [with T = bp_location; <template-parameter-1-2> = void; size_t = long unsigned int]’
  src/gdb/btrace.c:1153:42: error: use of deleted function ‘void* memset(T*, int, size_t) [with T = btrace_insn; <template-parameter-1-2> = void; size_t = long unsigned int]’
...

gdb/ChangeLog:
2017-04-25  Pedro Alves  <palves@redhat.com>

	* common/common-defs.h: Include "common/poison.h".
	* common/function-view.h: (Not, Or, Requires): Move to traits.h
	and adjust.
	* common/poison.h: New file.
	* common/traits.h: Include <type_traits>.
	(Not, Or, Requires): New, moved from common/function-view.h.