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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@88824 91177308-0d34-0410-b5e6-96231b3b80d8