Make it possible for AST plugins to enable themselves by default

Currently when an AST plugin is loaded it must then be enabled by passing
-plugin pluginname or -add-plugin pluginname to the -cc1 command line. This
patch adds a method to PluginASTAction which allows it to declare that the
action happens before, instead of, or after the main AST action, plus the
relevant changes to make the plugin action happen at that time automatically.

Differential Revision: http://reviews.llvm.org/D17959

llvm-svn: 263546