[NvlinkWrapper] Fix -pluing not consuming its argument

Summary:
Sometimes `clang` will pass `-plugin` when doing LTO, which should be
correctly consumed by the nvlink wrapper. Right now it was leaving the
`plugin.so` argument as a regular input, which would cause it to error
on the `.so` input.

clang/tools/clang-nvlink-wrapper/NVLinkOpts.td

@@ -39,7 +39,7 @@ def library_S : Separate<["--", "-"], "library">, Flags<[HelpHidden]>,
 def library_EQ : Joined<["--", "-"], "library=">, Flags<[HelpHidden]>,
   Alias<library_path>;
 
-def plugin : Joined<["--", "-"], "plugin">, 
+def plugin : JoinedOrSeparate<["--", "-"], "plugin">,
   Flags<[HelpHidden, WrapperOnlyOption]>;
 
 def arch : Separate<["--", "-"], "arch">,