[SIL] Separate thunks from signature optimization from other thunks.

Signature optimization is slightly different to (most) other thunks, in that
it's taking an existing function and turning that into a thunk, rather than
creating a thunk that calls an existing function. These symbols can be public,
etc. and so need to be handled a bit different to other types of thunks.

lib/SIL/SILPrinter.cpp

@@ -2326,6 +2326,9 @@ void SILFunction::print(SILPrintContext &PrintCtx) const {
   switch (isThunk()) {
   case IsNotThunk: break;
   case IsThunk: OS << "[thunk] "; break;
+  case IsSignatureOptimizedThunk:
+    OS << "[signature_optimized_thunk] ";
+    break;
   case IsReabstractionThunk: OS << "[reabstraction_thunk] "; break;
   }