tsan: add AccessVptr

Add AccessVptr access type.
For now it's converted to the same thr->is_vptr_access,
but later it will be passed directly to ReportRace
and will enable efficient tail calling in MemoryAccess function
(currently __tsan_vptr_update/__tsan_vptr_read can't use
tail calls in MemoryAccess because of the trailing assignment
to thr->is_vptr_access).

Depends on D107276.

Reviewed By: vitalybuka, melver

Differential Revision: https://reviews.llvm.org/D107282