[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.

include/swift/SIL/SILFunction.h

@@ -40,7 +40,12 @@ class SILProfiler;
 enum IsBare_t { IsNotBare, IsBare };
 enum IsTransparent_t { IsNotTransparent, IsTransparent };
 enum Inline_t { InlineDefault, NoInline, AlwaysInline };
-enum IsThunk_t { IsNotThunk, IsThunk, IsReabstractionThunk };
+enum IsThunk_t {
+  IsNotThunk,
+  IsThunk,
+  IsReabstractionThunk,
+  IsSignatureOptimizedThunk
+};
 
 class SILSpecializeAttr final {
   friend SILFunction;