Fix gold-plugin Windows build

r365588 missed one instance of integer file descriptor use in
gold-plugin.cpp.

llvm-svn: 366786
diff --git a/llvm/tools/gold/gold-plugin.cpp b/llvm/tools/gold/gold-plugin.cpp
index 1cd287c..1052c48 100644
--- a/llvm/tools/gold/gold-plugin.cpp
+++ b/llvm/tools/gold/gold-plugin.cpp
@@ -513,8 +513,8 @@
       offset = file->offset;
     }
     ErrorOr<std::unique_ptr<MemoryBuffer>> BufferOrErr =
-        MemoryBuffer::getOpenFileSlice(file->fd, file->name, file->filesize,
-                                       offset);
+        MemoryBuffer::getOpenFileSlice(sys::fs::convertFDToNativeFile(file->fd),
+                                       file->name, file->filesize, offset);
     if (std::error_code EC = BufferOrErr.getError()) {
       message(LDPL_ERROR, EC.message().c_str());
       return LDPS_ERR;