PR libstdc++/88782 avoid ODR problems in std::make_shared

PR libstdc++/88782 avoid ODR problems in std::make_shared

The old version of _Sp_counted_ptr_inplace::_M_get_deleter (up to GCC
8.2.0) expects to be passed a real std::typeinfo object, so mixing that
with the new definition of the __shared_ptr constructor (which always
passes the fake tag) leads to accessing the fake object as a real
std::typeinfo. Instead of trying to make it safe to mix the old and new
definitions, just stop using that function. By passing a reference to
__shared_ptr::_M_ptr to the __shared_count constructor it can be set
directly, without needing to obtain the pointer via the _M_get_deleter
back-channel. This avoids a virtual dispatch (which fixes PR 87514).

This means that code built against new libstdc++ headers doesn't use
_M_get_deleter at all, and so make_shared works the same whether RTTI is
enabled or not.

Also change _M_get_deleter so that it checks for a real type_info object
even when RTTI is disabled, by calling a library function. Unless
libstdc++ itself is built without RTTI that library function will be
able to test if it's the right type_info. This means the new definition
of _M_get_deleter can handle both the fake type_info tag and a real
type_info object, even if built without RTTI.

If linking to objects built against older versions of libstdc++ then if
all objects use -frtti or all use -fno-rtti, then the caller of
_M_get_deleter and the definition of _M_get_deleter will be consistent
and it will work. If mixing -frtti with -fno-rtti it can still fail if
the linker picks an old definition of _M_get_deleter and an old
__shared_ptr constructor that are incompatible. In that some or all
objects might need to be recompiled.

	PR libstdc++/87514
	PR libstdc++/87520
	PR libstdc++/88782
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.26): Export new symbol.
	* include/bits/shared_ptr.h
	(shared_ptr(_Sp_make_shared_tag, const Alloc&, Args&&...))
	(allocate_shared): Change to use new tag type.
	* include/bits/shared_ptr_base.h (_Sp_make_shared_tag::_S_eq):
	Declare new member function.
	(_Sp_alloc_shared_tag): Define new type.
	(_Sp_counted_ptr_inplace): Declare __shared_count<_Lp> as a friend.
	(_Sp_counted_ptr_inplace::_M_get_deleter) [!__cpp_rtti]: Use
	_Sp_make_shared_tag::_S_eq to check type_info.
	(__shared_count(Ptr, Deleter),__shared_count(Ptr, Deleter, Alloc)):
	Constrain to prevent being called with _Sp_alloc_shared_tag.
	(__shared_count(_Sp_make_shared_tag, const _Alloc&, Args&&...)):
	Replace constructor with ...
	(__shared_count(Tp*&, _Sp_alloc_shared_tag<_Alloc>, Args&&...)): Use
	reference parameter so address of the new object can be returned to
	the caller. Obtain the allocator from the tag type.
	(__shared_ptr(_Sp_make_shared_tag, const Alloc&, Args&&...)): Replace
	constructor with ...
	(__shared_ptr(_Sp_alloc_shared_tag<Alloc>, Args&&...)): Pass _M_ptr
	to the __shared_count constructor.
	(__allocate_shared): Change to use new tag type.
	* src/c++11/shared_ptr.cc (_Sp_make_shared_tag::_S_eq): Define.

From-SVN: r268086