Add correction message to bad linkage error.

src/IceTargetLoweringX86BaseImpl.h

@@ -1309,7 +1309,7 @@ void TargetX86Base<Machine>::lowerArithmetic(const InstArithmetic *Inst) {
       ++SwapCount;
     }
     assert(SwapCount <= 1);
-    (void) SwapCount;
+    (void)SwapCount;
   }
   if (!Traits::Is64Bit && Dest->getType() == IceType_i64) {
     // These x86-32 helper-call-involved instructions are lowered in this

src/PNaClTranslator.cpp

@@ -473,18 +473,23 @@ private:
     }
   }
 
-  // Converts function declarations into constant value IDs.
-  void createValueIDsForFunctions() {
-    Ice::GlobalContext *Ctx = getTranslator().getContext();
-    for (const Ice::FunctionDeclaration *Func : FunctionDeclarations) {
-      if (!Func->verifyLinkageCorrect(Ctx)) {
+  void reportLinkageError(const char *Kind,
+                          const Ice::GlobalDeclaration &Decl) {
     std::string Buffer;
     raw_string_ostream StrBuf(Buffer);
-        StrBuf << "Function " << Func->getName()
-               << " has incorrect linkage: " << Func->getLinkageName();
+    StrBuf << Kind << " " << Decl.getName()
+           << " has incorrect linkage: " << Decl.getLinkageName();
+    if (Decl.isExternal())
+      StrBuf << "\n  Use flag -allow-externally-defined-symbols to override";
     Error(StrBuf.str());
-        continue;
   }
+
+  // Converts function declarations into constant value IDs.
+  void createValueIDsForFunctions() {
+    Ice::GlobalContext *Ctx = getTranslator().getContext();
+    for (const Ice::FunctionDeclaration *Func : FunctionDeclarations) {
+      if (!Func->verifyLinkageCorrect(Ctx))
+        reportLinkageError("Function", *Func);
       Ice::Constant *C = nullptr;
       if (!isIRGenerationDisabled()) {
         C = getConstantSym(Func->getName(), Func->getSuppressMangling(),
@@ -498,13 +503,8 @@ private:
   void createValueIDsForGlobalVars() {
     Ice::GlobalContext *Ctx = getTranslator().getContext();
     for (const Ice::VariableDeclaration *Decl : *VariableDeclarations) {
-      if (!Decl->verifyLinkageCorrect(Ctx)) {
-        std::string Buffer;
-        raw_string_ostream StrBuf(Buffer);
-        StrBuf << "Global " << Decl->getName()
-               << " has incorrect linkage: " << Decl->getLinkageName();
-        Error(StrBuf.str());
-      }
+      if (!Decl->verifyLinkageCorrect(Ctx))
+        reportLinkageError("Global", *Decl);
       Ice::Constant *C = nullptr;
       if (!isIRGenerationDisabled()) {
         C = getConstantSym(Decl->getName(), Decl->getSuppressMangling(),