Add -Wdangling-pointer [PR63272].

Add -Wdangling-pointer [PR63272].

Resolves:
PR c/63272 - GCC should warn when using pointer to dead scoped variable with
in the same function

gcc/c-family/ChangeLog:

	PR c/63272
	* c.opt (-Wdangling-pointer): New option.

gcc/ChangeLog:

	PR c/63272
	* diagnostic-spec.c (nowarn_spec_t::nowarn_spec_t): Handle
	-Wdangling-pointer.
	* doc/invoke.texi (-Wdangling-pointer): Document new option.
	* gimple-ssa-warn-access.cc (pass_waccess::clone): Set new member.
	(pass_waccess::check_pointer_uses): New function.
	(pass_waccess::gimple_call_return_arg): New function.
	(pass_waccess::gimple_call_return_arg_ref): New function.
	(pass_waccess::check_call_dangling): New function.
	(pass_waccess::check_dangling_uses): New function overloads.
	(pass_waccess::check_dangling_stores): New function.
	(pass_waccess::check_dangling_stores): New function.
	(pass_waccess::m_clobbers): New data member.
	(pass_waccess::m_func): New data member.
	(pass_waccess::m_run_number): New data member.
	(pass_waccess::m_check_dangling_p): New data member.
	(pass_waccess::check_alloca): Check m_early_checks_p.
	(pass_waccess::check_alloc_size_call): Same.
	(pass_waccess::check_strcat): Same.
	(pass_waccess::check_strncat): Same.
	(pass_waccess::check_stxcpy): Same.
	(pass_waccess::check_stxncpy): Same.
	(pass_waccess::check_strncmp): Same.
	(pass_waccess::check_memop_access): Same.
	(pass_waccess::check_read_access): Same.
	(pass_waccess::check_builtin): Call check_pointer_uses.
	(pass_waccess::warn_invalid_pointer): Add arguments.
	(is_auto_decl): New function.
	(pass_waccess::check_stmt): New function.
	(pass_waccess::check_block): Call check_stmt.
	(pass_waccess::execute): Call check_dangling_uses,
	check_dangling_stores.  Empty m_clobbers.
	* passes.def (pass_warn_access): Invoke pass two more times.

gcc/testsuite/ChangeLog:

	PR c/63272
	* g++.dg/warn/Wfree-nonheap-object-6.C: Disable valid warnings.
	* g++.dg/warn/ref-temp1.C: Prune expected warning.
	* gcc.dg/uninit-pr50476.c: Expect a new warning.
	* c-c++-common/Wdangling-pointer-2.c: New test.
	* c-c++-common/Wdangling-pointer-3.c: New test.
	* c-c++-common/Wdangling-pointer-4.c: New test.
	* c-c++-common/Wdangling-pointer-5.c: New test.
	* c-c++-common/Wdangling-pointer-6.c: New test.
	* c-c++-common/Wdangling-pointer.c: New test.
	* g++.dg/warn/Wdangling-pointer-2.C: New test.
	* g++.dg/warn/Wdangling-pointer.C: New test.
	* gcc.dg/Wdangling-pointer-2.c: New test.
	* gcc.dg/Wdangling-pointer.c: New test.