[LinkerTest] Use LLVMDisposeMessage to free error string.

LLVMDisposeMessage is just a thing wrapper around free at the moment, but it's
the proper API to use here.

llvm-svn: 239731
diff --git a/llvm/unittests/Linker/LinkModulesTest.cpp b/llvm/unittests/Linker/LinkModulesTest.cpp
index 2a0a7bf..45f1308 100644
--- a/llvm/unittests/Linker/LinkModulesTest.cpp
+++ b/llvm/unittests/Linker/LinkModulesTest.cpp
@@ -216,7 +216,7 @@
                                     LLVMLinkerDestroySource, &errout);
   EXPECT_EQ(1, result);
   EXPECT_STREQ("Linking globals named 'foo': symbol multiply defined!", errout);
-  std::free(errout);
+  LLVMDisposeMessage(errout);
 }
 
 } // end anonymous namespace