[WholeProgramDevirt] Add check for AvailableExternal and give up icall.branch.funnel (#143468) 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.
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