Fully ignore inappropriately named loplugins in unit-test mode

...even if they implement PPCallbacks, so filtering them out in
HandleTranslationUnit was ineffective.

Change-Id: I9df8103a50739f3176e6d63accfd0334da7faa9a
Reviewed-on: https://gerrit.libreoffice.org/43575
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/pluginhandler.hxx

@@ -43,6 +43,7 @@ private:
     void createPlugins( std::set< std::string > rewriters );
     DiagnosticBuilder report( DiagnosticsEngine::Level level, StringRef message, SourceLocation loc = SourceLocation());
     CompilerInstance& compiler;
+    StringRef const mainFileName;
     Rewriter rewriter;
     std::set< SourceLocation > removals;
     std::string scope;