When a customer class inherits from a libc++ class, and is built with
"-flto -fwhole-program-vtables -static-libstdc++ \
-Wl,-plugin-opt=-whole-program-visibility", the libc++ class's vtable is
available_externally, meanwhile the customer class vtable is private.
And
both of them are !vcall_visibility == Linkage Unit.
In this case, icall.branch.funnel might be generated.
But the icall.branch.funnel would cause crash in LowerTypeTests because
available_externally Global_Object's GlobalTypeMember would not be
saved and finally leads to a NULL GlobalTypeMember which causes a crash.
Even saving the available_externally GO's GlobalTypeMember so that it is
not NULL to avoid the crash in LowerTypeTests, it still will crash in
SelectionDAGBuilder or Verifier, because operands linkage type
consistency
check of icall.branch.funnel can not pass.
So any one of available externally vtable would stop to generate
icall.branch.funnel.
This patch fixes FullLTO mode and split-LTO-unit ThinLTO mode.
6001a8bb94