Print the correct error when our DynamicCheckerFunctions fail to install

[teemperor]

Summary

The check is inverted here: If we have error messages, we should print those instead
of our default error message. But currently we print the default message when we
actually have a sensible error to print.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38383
Thanks Nat for the patch!

[teemperor]

Don't see a way to test this, as this can only be triggered when something in LLDB is going really wrong. That piece of code also has zero test coverage, so I'll just merge it in a few weeks or so unless someone comes up with a good way to test this.

[JDevlieghere]

LGTM!

lldb/trunk/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

if (execution_policy == eExecutionPolicyAlways ||
         if (!process->GetDynamicCheckers()) {
           DynamicCheckerFunctions *dynamic_checkers =
               new DynamicCheckerFunctions();
 
           DiagnosticManager install_diagnostics;
 
           if (!dynamic_checkers->Install(install_diagnostics, exe_ctx)) {
             if (install_diagnostics.Diagnostics().size())
-              err.SetErrorString("couldn't install checkers, unknown error");
-            else
               err.SetErrorString(install_diagnostics.GetString().c_str());
+            else
+              err.SetErrorString("couldn't install checkers, unknown error");
 
             return err;
           }
 
           process->SetDynamicCheckers(dynamic_checkers);
 
           if (log)
             log->Printf("== [ClangUserExpression::Evaluate] Finished "