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/FrontendTool/FrontendTool.cpp

@@ -625,6 +625,9 @@ static bool performCompile(std::unique_ptr<CompilerInstance> &Instance,
   if (Invocation.getSILOptions().LinkMode == SILOptions::LinkAll)
     performSILLinking(SM.get(), true);
 
+  if (Invocation.getSILOptions().MergePartialModules)
+    SM->linkAllFromCurrentModule();
+
   {
     SharedTimer timer("SIL verification, pre-optimization");
     SM->verify();