Add a new -assume-single-threaded option

This is a hidden option. It should be used like: -assume-single-threaded

When this function is provided, the compiler assumes that the code will be executed in the single threaded mode. It then performs certain optimizations that can benefit from it, e.g. it  marks as non-atomic all reference counting instructions in the user code being compiled.

lib/Frontend/CompilerInvocation.cpp

@@ -1114,6 +1114,10 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
     }
   }
 
+  if (Args.getLastArg(OPT_AssumeSingleThreaded)) {
+    Opts.AssumeSingleThreaded = true;
+  }
+
   // Parse the assert configuration identifier.
   if (const Arg *A = Args.getLastArg(OPT_AssertConfig)) {
     StringRef Configuration = A->getValue();