Subzero: Add -allow-extern as an alias for --allow-externally-defined-symbols.

src/IceConverter.cpp

Index: src/IceConverter.cpp
diff --git a/src/IceConverter.cpp b/src/IceConverter.cpp
index 349fde54f676cabee6ff034bd856d333db6d1439..a84ae2cf69d4e2004ed508f82305a148c82619e7 100644
--- a/src/IceConverter.cpp
+++ b/src/IceConverter.cpp
@@ -872,6 +872,8 @@ void Converter::installGlobalDeclarations(Module *Mod) {
       raw_string_ostream StrBuf(Buffer);
       StrBuf << "Function " << IceFunc->getName()
              << " has incorrect linkage: " << IceFunc->getLinkageName();
+      if (IceFunc->isExternal())
+        StrBuf << "\n  Use flag -allow-externally-defined-symbols to override";
       report_fatal_error(StrBuf.str());
     }
     GlobalDeclarationMap[&Func] = IceFunc;
@@ -891,6 +893,8 @@ void Converter::installGlobalDeclarations(Module *Mod) {
       raw_string_ostream StrBuf(Buffer);
       StrBuf << "Global " << Var->getName()
              << " has incorrect linkage: " << Var->getLinkageName();
+      if (Var->isExternal())
+        StrBuf << "\n  Use flag -allow-externally-defined-symbols to override";
       report_fatal_error(StrBuf.str());
     }
     GlobalDeclarationMap[GV] = Var;