[llvm-strip] Hook up (unimplemented) --only-keep-debug

For ELF, we accept but ignore --only-keep-debug. Do the same for llvm-strip.

COFF does implement this, so update the test that it is supported.

llvm-svn: 356207
diff --git a/llvm/tools/llvm-objcopy/CopyConfig.cpp b/llvm/tools/llvm-objcopy/CopyConfig.cpp
index 4786782..dd3fa3e 100644
--- a/llvm/tools/llvm-objcopy/CopyConfig.cpp
+++ b/llvm/tools/llvm-objcopy/CopyConfig.cpp
@@ -725,6 +725,7 @@
   Config.StripUnneeded = InputArgs.hasArg(STRIP_strip_unneeded);
   Config.StripAll = InputArgs.hasArg(STRIP_strip_all);
   Config.StripAllGNU = InputArgs.hasArg(STRIP_strip_all_gnu);
+  Config.OnlyKeepDebug = InputArgs.hasArg(STRIP_only_keep_debug);
   Config.KeepFileSymbols = InputArgs.hasArg(STRIP_keep_file_symbols);
 
   for (auto Arg : InputArgs.filtered(STRIP_keep_section))