Add support for plugins add passes to the default set of passes. The standard...

Add support for plugins add passes to the default set of passes.  The standard set of passes used by front ends can now be modified by LLVM plugins, without needing to modify any front ends.

Still to do:

- Allow replacing / removing passes (infrastructure there, just needs an infrastructure exposed)
- Defining sets of passes to be added or removed as a group
- Extending the support to allow user-defined groups of optimisations
- Allow plugins to be specified for loading automatically (e.g. from plugins.conf or some similar mechanism)

Reviewed by Nick Lewycky.

llvm-svn: 131155