Frontend: Add a new -sil-merge-partial-modules flag

Previously we would drop all serialized SIL from partial swiftmodule
files generated while compiling source in non-WMO mode; all that was
missing was linking it in.

This adds a frontend flag, and a test; driver change is coming up
next.

Progress on <rdar://problem/18913977>.

lib/Frontend/CompilerInvocation.cpp

@@ -1218,6 +1218,9 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
       llvm_unreachable("Unknown SIL linking option!");
   }
 
+  if (Args.hasArg(OPT_sil_merge_partial_modules))
+    Opts.MergePartialModules = true;
+
   // Parse the optimization level.
   if (const Arg *A = Args.getLastArg(OPT_O_Group)) {
     if (A->getOption().matches(OPT_Onone)) {