[clang][objc] Fix non-deterministic stub fixit output for missing requirements

When outputting fixits for missing methods, we had two issues
* If we found an optional method we could incorrectly hide the fixit for
  a requiremed method with the same selector from another protocol
* The fixit order wasn't deterministic when two protocols declared the
  same required selector

Fix by choosing the best candidate to store when walking methods.

rdar://109411920
(cherry picked from commit 635f5a6)

Author: benlangmuir

clang/lib/Edit/FillInMissingProtocolStubs.cpp

@@ -145,7 +145,20 @@ class MethodSet {
       if (Availability == AR_NotYetIntroduced || Availability == AR_Unavailable)
         continue;
       auto &Map = M->isInstanceMethod() ? InstanceMethods : ClassMethods;
-      Map.insert(std::make_pair(M->getSelector(), MethodInfo(M, P, Priority)));
+      MethodInfo MI(M, P, Priority);
+      auto [I, Inserted] = Map.insert(std::make_pair(M->getSelector(), MI));
+      if (Inserted)
+        continue;
+
+      // Prefer a required method so we do not miss fixits, and the lowest
+      // priority so the order of fixits will be deterministic.
+      if (MI.isRequired() != I->second.isRequired()) {
+        if (MI.isRequired())
+          I->second = MI;
+        continue;
+      }
+      if (MI.ProtocolPriority < I->second.ProtocolPriority)
+        I->second = MI;
     }
   }
 

clang/test/FixIt/fixit-fill-in-protocol-requirements.m

@@ -82,3 +82,32 @@ @implementation FixitJustAvailable // expected-warning {{class 'FixitJustAvailab
 @end
 // CHECK: fix-it:{{.*}}:{[[@LINE-1]]:1-[[@LINE-1]]:1}:"- (void)availableMethod { \n  <#code#>\n}\n\n- (void)deprecatedMethod { \n  <#code#>\n}\n\n"
 // AVAILABLE: fix-it:{{.*}}:{[[@LINE-2]]:1-[[@LINE-2]]:1}:"- (void)availableMethod { \n  <#code#>\n}\n\n- (void)deprecatedMethod { \n  <#code#>\n}\n\n+ (void)notYetAvailableMethod { \n  <#code#>\n}\n\n"
+
+@protocol PReq1
+-(void)reqZ1;
+@end
+
+@protocol PReq2
+-(void)reqZ1;
+-(void)reqA2;
+-(void)reqA1;
+@end
+
+// Ensure optional cannot hide required methods with the same selector.
+@protocol POpt
+@optional
+-(void)reqZ1;
+-(void)reqA2;
+-(void)reqA1;
+@end
+
+@interface MultiReqOpt <PReq1, PReq2, POpt>
+@end
+@implementation MultiReqOpt // expected-warning {{class 'MultiReqOpt' does not conform to protocols 'PReq1' and 'PReq2'}} expected-note {{add stubs}}
+@end
+// CHECK: fix-it:{{.*}}:{[[@LINE-1]]:1-[[@LINE-1]]:1}:"
+// Z1 is first due to being from the first listed protocol, PReq1
+// CHECK-SAME: - (void)reqZ1
+// A1 is before A2 because of secondary sort by name.
+// CHECK-SAME: - (void)reqA1
+// CHECK-SAME: - (void)reqA2