c++: Further fix for -fsanitize=vptr [PR94325]

For -fsanitize=vptr, we insert a NULL store into the vptr instead of just
adding a CLOBBER of this.  build_clobber_this makes the CLOBBER conditional
on in_charge (implicit) parameter whenever CLASSTYPE_VBASECLASSES, but when
adding this conditionalization to the -fsanitize=vptr code in PR87095,
I wanted it to catch some more cases when the class has CLASSTYPE_VBASECLASSES,
but the vptr is still not shared with something else, otherwise the
sanitization would be less effective.
The following testcase shows that the chosen test that CLASSTYPE_PRIMARY_BINFO
is non-NULL and has BINFO_VIRTUAL_P set wasn't sufficient,
the D class has still sizeof(D) == sizeof(void*) and thus contains just
a single vptr, but while in B::~B() this results in the vptr not being
cleared, in C::~C() this condition isn't true, as CLASSTYPE_PRIMARY_BINFO
in that case is B and is not BINFO_VIRTUAL_P, so it clears the vptr, but the
D::~D() dtor after invoking C::~C() invokes A::~A() with an already cleared
vptr, which is then reported.
The following patch is just a shot in the dark, keep looking through
CLASSTYPE_PRIMARY_BINFO until we find BINFO_VIRTUAL_P, but it works on the
existing testcase as well as this new one.

2020-04-08  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94325
	* decl.c (begin_destructor_body): For CLASSTYPE_VBASECLASSES class
	dtors, if CLASSTYPE_PRIMARY_BINFO is non-NULL, but not BINFO_VIRTUAL_P,
	look at CLASSTYPE_PRIMARY_BINFO of its BINFO_TYPE if it is not
	BINFO_VIRTUAL_P, and so on.

	* g++.dg/ubsan/vptr-15.C: New test.

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