Avoid parsing -mllvm* command-line options if there aren't any

LLD somehow uses a different global LLVM command-line parser than LDC,
one with no registered options, so parsing is guaranteed to fail, even
if there are no args.
[It's only needed for LTO codegen options anyway.]

Author: kinke

COFF/Driver.cpp

@@ -746,7 +746,8 @@ void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   V.push_back("lld-link (LLVM option parsing)");
   for (auto *Arg : Args.filtered(OPT_mllvm))
     V.push_back(Arg->getValue());
-  cl::ParseCommandLineOptions(V.size(), V.data());
+  if (V.size() > 1)
+    cl::ParseCommandLineOptions(V.size(), V.data());
 
   // Handle /errorlimit early, because error() depends on it.
   if (auto *Arg = Args.getLastArg(OPT_errorlimit)) {

ELF/Driver.cpp

@@ -259,7 +259,8 @@ static void initLLVM(opt::InputArgList &Args) {
   V.push_back("lld (LLVM option parsing)");
   for (auto *Arg : Args.filtered(OPT_mllvm))
     V.push_back(Arg->getValue());
-  cl::ParseCommandLineOptions(V.size(), V.data());
+  if (V.size() > 1)
+    cl::ParseCommandLineOptions(V.size(), V.data());
 }
 
 // Some command line options or some combinations of them are not allowed.