[llvm] [WholeProgramDevirt] Add check for AvailableExternal and give up devirt (PR #143468)

[Shilei Tian via llvm-commits]

================
@@ -1093,6 +1093,7 @@ bool DevirtModule::tryFindVirtualCallTargets(
     std::vector<VirtualCallTarget> &TargetsForSlot,
     const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset,
     ModuleSummaryIndex *ExportSummary) {
+  bool hasAvailableExternally = false;
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shiltian wrote:

```suggestion
  bool HasAvailableExternally = false;
```

https://github.com/llvm/llvm-project/pull/143468