[clang] Don't clear AST if we have consumers running after the main action

Downstream users may have Clang plugins. By default these plugins run
after the main action if they are specified on the command line.

Since these plugins are ASTConsumers, presumably they inspect the AST.
So we shouldn't clear it if any plugins run after the main action.

Reviewed By: dblaikie, hans

Differential Revision: https://reviews.llvm.org/D112190