Fix Registry::import() crash on mingw when loading a DLL plugin

[Ilod]

Summary

Fix support for importing and exporting Registry objects on Windows

On Windows, most compilers will produce a DLL with their own global Tail and Head variables, so we copy these variables returned from an exported function of the DLLs into the consumer Registry, but some (such as MinGW) shares it with the consumer (same as on other platforms).

Add a check before the copy to skip it if we detect the Head returned is the same as the consumer one, avoiding to copy it into itself, which is an infinite copy.
Also add null-check to avoid crash when exporting a DLL with no plugin registered.

Edit CMake rules to allow compiling example plugins on Windows.

[Ilod]

Updated to head. Removed the Cmake rule modification (because we need first to add some code in clang to really handle plugins on Windows).

To be more precise: the crash occurs on Windows clang built with MinGW if r260265 is applied and a plugin is used. r260265 has been reverted, and we need this patch before reapplying it to have support of Windows DLL plugins.

cmake/modules/HandleLLVMOptions.cmake

append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
                  CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 elseif(LLVM_ENABLE_LTO)
   append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
                  CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 
 # Plugin support
 # FIXME: Make this configurable.
-if(WIN32 OR CYGWIN)
-  if(BUILD_SHARED_LIBS)
-    set(LLVM_ENABLE_PLUGINS ON)
-  else()
-    set(LLVM_ENABLE_PLUGINS OFF)
-  endif()
-else()
-  set(LLVM_ENABLE_PLUGINS ON)
-endif()

include/llvm/Support/Registry.h

static void import(sys::DynamicLibrary &DL, const char *RegistryName) {
       GetRegistry Getter =
           reinterpret_cast<GetRegistry>(DL.getAddressOfSymbol(Name.c_str()));
       if (Getter) {
         // Call the getter function in order to get the full copy of the
         // registry defined in the plugin DLL, and copy them over to the
         // current Registry.
         typedef std::pair<const node *, const node *> Info;
         Info *I = static_cast<Info *>(Getter());
+        // We need to check I->first != Head for mingw, which build DLLs
+        // sharing extern symbols when using LLVM_BUILD_SHARED, so we don't
+        // copy the Registry in itself.
+        if (I->first != nullptr && I->first != Head) {
           iterator begin(I->first);
           iterator end(I->second);
           for (++end; begin != end; ++begin) {
             // This Node object needs to remain alive for the
             // duration of the program.
             add_node(new node(*begin));
           }
         }
       }
+    }
 
     /// Retrieve the data to be passed across DLL boundaries when
     /// importing registries from another DLL on Windows.
     static void *exportRegistry() {
       static std::pair<const node *, const node *> Info(Head, Tail);
       return &Info;
     }
   };