Fix locking for printing error messages.

unittest/IceParseInstsTest.cpp

Index: unittest/IceParseInstsTest.cpp
diff --git a/unittest/IceParseInstsTest.cpp b/unittest/IceParseInstsTest.cpp
index 5d45888dd22471ce8f19628e6152921714d62328..191eff9203a4ad6583d173e781ec507dfa92d8ac 100644
--- a/unittest/IceParseInstsTest.cpp
+++ b/unittest/IceParseInstsTest.cpp
@@ -63,6 +63,12 @@ TEST(IceParseInstsTest, NonexistentCallArg) {
   EXPECT_FALSE(Munger.runTest("Nonexistent call arg"));
   EXPECT_EQ("Error(66:4): Invalid function record: <34 0 4 2 100>\n",
             Munger.getTestResults());
+
+  // Show that we generate a fatal error when not allowing error recovery.
+  Munger.Flags.setAllowErrorRecovery(false);
+  EXPECT_DEATH(
+      Munger.runTest("Nonexistent call arg"),
+      ".*ERROR: Unable to continue.*");
 }
 
 /// Test how we recognize alignments in alloca instructions.