[flang][plugins] Make PluginParseTreeAction an abstract class

There's no point in providing a default implementation for
`PluginParseTreeAction`. This patch makes it abstract forcing users to
specialise it in order to use it.

Differential Revision: https://reviews.llvm.org/D108518

flang/include/flang/Frontend/FrontendActions.h

@@ -128,7 +128,7 @@ class ParseSyntaxOnlyAction : public PrescanAndSemaAction {
 };
 
 class PluginParseTreeAction : public PrescanAndSemaAction {
-  void ExecuteAction() override;
+  void ExecuteAction() override = 0;
 };
 
 } // namespace Fortran::frontend