[llvm-ar] Ignore -plugin option.

binutils ar uses -plugin to specify the LTO plugin, but LLVM doesn't
need this as it doesn't use a plugin for LTO. Accepting (and ignoring)
the option allows interoperability with existing build systems and
make downstream consumers life much easier.

No objections from Rafael on this change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273938 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-ar/llvm-ar.cpp

@@ -71,6 +71,7 @@ static cl::list<std::string>
                cl::desc("[relpos] [count] <archive-file> [members]..."));
 
 static cl::opt<bool> MRI("M", cl::desc(""));
+static cl::opt<std::string> Plugin("plugin", cl::desc("plugin (ignored for compatibility"));
 
 namespace {
 enum Format { Default, GNU, BSD };