Instantiate llvm::Registry<clang::PluginASTAction> in FrontendAction.cpp.

  - Plugins don't need to export _ZN4llvm8Registry*.
  - Win32.DLL cannot merge common symbols among DLLs. Static members in llvm::Registry should be instantiated in a parent.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212821 91177308-0d34-0410-b5e6-96231b3b80d8