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/SIL/SILModule.cpp

@@ -568,6 +568,11 @@ bool SILModule::hasFunction(StringRef Name) {
   return Visitor.hasFunction(Name);
 }
 
+void SILModule::linkAllFromCurrentModule() {
+  getSILLoader()->getAllForModule(getSwiftModule()->getName(),
+                                  /*PrimaryFile=*/nullptr);
+}
+
 void SILModule::linkAllWitnessTables() {
   getSILLoader()->getAllWitnessTables();
 }