Revert D135427 "[LTO] Make local linkage GlobalValue in non-prevailing COMDAT available_externally"

This reverts commit 8901635423cbea4324059a5127657126d2e00ce1.

This change broke the following example and we need to check `if (GO->getComdat()->getName() == GO->getName())`
before `NonPrevailingComdats.insert(GO->getComdat());`
Revert for clarify.

```
// a.cc
template <typename T>
struct A final { virtual ~A() {} };
extern "C" void aa() { A<int> a; }
// b.cc
template <typename T>
struct A final { virtual ~A() {} };
template struct A<int>;
extern "C" void bb(A<int> *a) { delete a; }

clang -c -fpic -O0 -flto=thin a.cc && ld.lld -shared a.o b.o
```

1 files changed, 5 insertions, 5 deletions

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 9bfbabc..d3f29a6 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -712,11 +712,11 @@ handleNonPrevailingComdat(GlobalValue &GV,
   if (!NonPrevailingComdats.count(C))
     return;
 
-  // Additionally need to drop all global values from the comdat to
-  // available_externally, to satisfy the COMDAT requirement that all members
-  // are discarded as a unit. The non-local linkage global values avoid
-  // duplicate definition linker errors.
-  GV.setLinkage(GlobalValue::AvailableExternallyLinkage);
+  // Additionally need to drop externally visible global values from the comdat
+  // to available_externally, so that there aren't multiply defined linker
+  // errors.
+  if (!GV.hasLocalLinkage())
+    GV.setLinkage(GlobalValue::AvailableExternallyLinkage);
 
   if (auto GO = dyn_cast<GlobalObject>(&GV))
     GO->setComdat(nullptr);