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

Also remind the user of that option in IceConverter.cpp, similar to PNaClTranslator.cpp.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1408023004 .

src/IceClFlags.cpp

diff --git a/src/IceClFlags.cpp b/src/IceClFlags.cpp
index 8fd50c7..7ee5228 100644
--- a/src/IceClFlags.cpp
+++ b/src/IceClFlags.cpp
@@ -40,6 +40,10 @@
              "and __pnacl_pso_root)."),
     cl::init(false));
 
+cl::alias AllowExternDefinedSymbolsA(
+    "allow-extern", cl::desc("Alias for --allow-externally-defined-symbols"),
+    cl::NotHidden, cl::aliasopt(AllowExternDefinedSymbols));
+
 cl::opt<bool> AllowIacaMarks(
     "allow-iaca-marks",
     cl::desc("Allow IACA (Intel Architecture Code Analyzer) marks to be "
@@ -216,7 +220,7 @@
 
 cl::opt<std::string> VerboseFocusOn(
     "verbose-focus",
-    cl::desc("Temporarily enable full verbosity for a specific function"),
+    cl::desc("Override with -verbose=none except for the specified function"),
     cl::init(""));
 
 cl::opt<Ice::FileType> OutFileType(

src/IceConverter.cpp

diff --git a/src/IceConverter.cpp b/src/IceConverter.cpp
index 349fde5..a84ae2c 100644
--- a/src/IceConverter.cpp
+++ b/src/IceConverter.cpp
@@ -872,6 +872,8 @@
       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 @@
       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;