Fix isDiscardableIfUnused to include available_externally linkage.

[eugenis]

Summary

AvailableExternally functions are discardable.

[eugenis]

I'm not sure how to test this. It does not break any existing tests, and the tests in the upcoming D12087 depend on this.

[rsmith]

LGTM, but I'd like someone with more familiarity with this to double-check there's no subtlety we're missing here.

[rnk]

lgtm

[eugenis]

Thanks, committed as r247044.

include/llvm/IR/GlobalValue.h

public:
   }
   static bool isCommonLinkage(LinkageTypes Linkage) {
     return Linkage == CommonLinkage;
   }
 
   /// Whether the definition of this global may be discarded if it is not used
   /// in its compilation unit.
   static bool isDiscardableIfUnused(LinkageTypes Linkage) {
-    return isLinkOnceLinkage(Linkage) || isLocalLinkage(Linkage);
+    return isLinkOnceLinkage(Linkage) || isLocalLinkage(Linkage) ||
+           isAvailableExternallyLinkage(Linkage);
   }
 
   /// Whether the definition of this global may be replaced by something
   /// non-equivalent at link time. For example, if a function has weak linkage
   /// then the code defining it may be replaced by different code.
   static bool mayBeOverridden(LinkageTypes Linkage) {
     return Linkage == WeakAnyLinkage || Linkage == LinkOnceAnyLinkage ||
            Linkage == CommonLinkage || Linkage == ExternalWeakLinkage;