[IRGen] Fix non-deterministic requests in addAbstractForFulfillments()

In `addAbstractForFulfillments()`, FulfillmentMap (DenseMap) is
iterated to compute all the cache entries. Even the
non-deterministic iteration order doesn't affect the computed cache
entries, the requests to the evaluator during the process will be in
non-deterministic order. This will cause the fine-grain dependency file
generated to be non-deterministic. Use a `MapVector` to make sure
iteration order is deterministic.

rdar://147971980

Author: cachemeifyoucan

lib/IRGen/Fulfillment.h

@@ -18,9 +18,9 @@
 #ifndef SWIFT_IRGEN_FULFILLMENT_H
 #define SWIFT_IRGEN_FULFILLMENT_H
 
-#include "llvm/ADT/DenseMap.h"
-#include "swift/AST/Types.h"
+#include "llvm/ADT/MapVector.h"
 #include "swift/AST/GenericSignature.h"
+#include "swift/AST/Types.h"
 #include "swift/IRGen/GenericRequirement.h"
 #include "MetadataPath.h"
 
@@ -49,7 +49,7 @@ struct Fulfillment {
 };
 
 class FulfillmentMap {
-  llvm::DenseMap<GenericRequirement, Fulfillment> Fulfillments;
+  llvm::MapVector<GenericRequirement, Fulfillment> Fulfillments;
 
 public:
   struct InterestingKeysCallback {