SIL: Set the context generic params of reabstraction thunks less greedily.

Don't try to set the generic params of a reabstraction thunk until we've established it hasn't already been defined, fixing an issue where the same generic reabstraction pattern occurred in different contexts and we ended up clobbering the archetypes of a thunk that was already defined in terms of other archetypes.

Swift SVN r13964

include/swift/SIL/SILModule.h

@@ -282,7 +282,6 @@ public:
   SILFunction *getOrCreateSharedFunction(SILLocation loc,
                                          StringRef name,
                                          CanSILFunctionType type,
-                                         GenericParamList *contextGenericParams,
                                          IsBare_t isBareSILFunction,
                                          IsTransparent_t isTransparent);