clang-cl: Support loading plugins on Windows

This builds on the support being added to LLVM to import and export
registries from DLLs.  This will allow us to pick up the registry
entries added in the DLL's copy of FrontendPluginRegistry.

This will allow us to use plugins on Windows using:
$ clang-cl -Xclang -load -Xclang plugin.dll \
           -Xclang -add-plugin -Xclang foo

llvm-svn: 260265