libclang: expose dllexport, dllimport attributes

These attributes were previously unexposed.  Expose them through the libclang
interfaces.  Add tests that cover both the MSVC spelling and the GNU spelling.

llvm-svn: 255273
diff --git a/clang/tools/libclang/CXCursor.cpp b/clang/tools/libclang/CXCursor.cpp
index fbf0aaf..a980981 100644
--- a/clang/tools/libclang/CXCursor.cpp
+++ b/clang/tools/libclang/CXCursor.cpp
@@ -59,6 +59,8 @@
     case attr::CUDAHost: return CXCursor_CUDAHostAttr;
     case attr::CUDAShared: return CXCursor_CUDASharedAttr;
     case attr::Visibility: return CXCursor_VisibilityAttr;
+    case attr::DLLExport: return CXCursor_DLLExport;
+    case attr::DLLImport: return CXCursor_DLLImport;
   }
 
   return CXCursor_UnexposedAttr;