[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/Serialization/SILFormat.h

@@ -286,7 +286,7 @@ namespace sil_block {
       BCRecordLayout<SIL_FUNCTION, SILLinkageField,
                      BCFixed<1>,  // transparent
                      BCFixed<2>,  // serialized
-                     BCFixed<2>,  // thunk/reabstraction_thunk
+                     BCFixed<2>,  // thunks: signature optimized/reabstraction
                      BCFixed<1>,  // global_init
                      BCFixed<2>,  // inlineStrategy
                      BCFixed<2>,  // optimizationMode