Add a mechanism for optimisation plugins to register passes that all front ends can use without needing to be aware of the plugin (or the plugin be aware of the front end).

Before 3.0, I'd like to add a mechanism for automatically loading a set of plugins from a config file.  API suggestions welcome...

llvm-svn: 137717

functionality since in the C api a pass is created and added to a pass
manager in a single call.

llvm-svn: 137159

llvm-svn: 136727