Allow to load DLL plugins on MSVC clang

[Ilod]

Summary

This is a resubmit of r260265, adding support for PragmaHandler plugins, which depends on a crash fix on LLVM (D18847)

docs/ClangPlugins.rst

bool ParseArgs(const CompilerInstance &CI,
     }
     return true;
   }
 
 Registering a plugin
 ====================
 
 A plugin is loaded from a dynamic library at runtime by the compiler. To
-register a plugin in a library, use ``FrontendPluginRegistry::Add<>``:
+register a plugin in a library, use ``FrontendPluginRegistry::Add<>``.
+On Windows, you also need to export your plugin registry using
+``LLVM_EXPORT_REGISTRY``.  Here is an example:
 
 .. code-block:: c++
 
   static FrontendPluginRegistry::Add<MyPlugin> X("my-plugin-name", "my plugin description");
+  LLVM_EXPORT_REGISTRY(FrontendPluginRegistry)
 
 Defining pragmas
 ================
 
 Plugins can also define pragmas by declaring a ``PragmaHandler`` and
 registering it using ``PragmaHandlerRegistry::Add<>``:
 
 .. code-block:: c++

examples/AnnotateFunctions/AnnotateFunctions.cpp

 
 }
 
 static FrontendPluginRegistry::Add<AnnotateFunctionsAction>
 X("annotate-fns", "annotate functions");
 
 static PragmaHandlerRegistry::Add<PragmaAnnotateHandler>
 Y("enable_annotate","enable annotation");
+
+LLVM_EXPORT_REGISTRY(FrontendPluginRegistry)
+LLVM_EXPORT_REGISTRY(PragmaHandlerRegistry)

examples/PrintFunctionNames/PrintFunctionNames.cpp

protected:
   }
 
 };
 
 }
 
 static FrontendPluginRegistry::Add<PrintFunctionNamesAction>
 X("print-fns", "print function names");
+LLVM_EXPORT_REGISTRY(FrontendPluginRegistry)

lib/FrontendTool/ExecuteCompilerInvocation.cpp

 #include "clang/CodeGen/CodeGenAction.h"
 #include "clang/Driver/Options.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Frontend/FrontendDiagnostic.h"
 #include "clang/Frontend/FrontendPluginRegistry.h"
 #include "clang/Frontend/Utils.h"
+#include "clang/Lex/Preprocessor.h"
 #include "clang/Rewrite/Frontend/FrontendActions.h"
 #include "clang/StaticAnalyzer/Frontend/FrontendActions.h"
 #include "llvm/Option/OptTable.h"
 #include "llvm/Option/Option.h"
 #include "llvm/Support/DynamicLibrary.h"
 #include "llvm/Support/ErrorHandling.h"
 using namespace clang;
 using namespace llvm::opt;
if (Clang->getFrontendOpts().ShowVersion) {
     return true;
   }
 
   // Load any requested plugins.
   for (unsigned i = 0,
          e = Clang->getFrontendOpts().Plugins.size(); i != e; ++i) {
     const std::string &Path = Clang->getFrontendOpts().Plugins[i];
     std::string Error;
-    if (llvm::sys::DynamicLibrary::LoadLibraryPermanently(Path.c_str(), &Error))
+    llvm::sys::DynamicLibrary DL(
+        llvm::sys::DynamicLibrary::getPermanentLibrary(Path.c_str(), &Error));
+    if (DL.isValid()) {
+      // On Windows, we need to import the plugin registries dynamically.
+      LLVM_IMPORT_REGISTRY(FrontendPluginRegistry, DL);
+      LLVM_IMPORT_REGISTRY(PragmaHandlerRegistry, DL);
+    } else {
       Clang->getDiagnostics().Report(diag::err_fe_unable_to_load_plugin)
         << Path << Error;
     }
+  }
 
   // Check if any of the loaded plugins replaces the main AST action
   for (FrontendPluginRegistry::iterator it = FrontendPluginRegistry::begin(),
                                         ie = FrontendPluginRegistry::end();
        it != ie; ++it) {
     std::unique_ptr<PluginASTAction> P(it->instantiate());
     if (P->getActionType() == PluginASTAction::ReplaceAction) {
       Clang->getFrontendOpts().ProgramAction = clang::frontend::PluginAction;