[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239731 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Linker/LinkModulesTest.cpp

@@ -216,7 +216,7 @@ TEST_F(LinkModuleTest, CAPIFailure) {
                                     LLVMLinkerDestroySource, &errout);
   EXPECT_EQ(1, result);
   EXPECT_STREQ("Linking globals named 'foo': symbol multiply defined!", errout);
-  std::free(errout);
+  LLVMDisposeMessage(errout);
 }
 
 } // end anonymous namespace