Add pluggable action support to clang-cc, via -plugin command line option.

 - Expects the plugin has been loaded with -load.

 - Using this may require disabling TOOL_NO_EXPORTS in the clang-cc Makefile, this breaks the llvm::Registry way of working (static constructors are bad, kids). This should be replaced with a "real" plugin model that has explicit plugin interfaces.

llvm-svn: 88824