[RequirementMachine] Omit duplicate conflict diagnostics.

Author: hborla

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -796,6 +796,10 @@ void RewriteSystem::computeConflictDiagnostics(
 
     assert(firstRule.isPropertyRule() && secondRule.isPropertyRule());
 
+    if (firstRule.isSubstitutionSimplified() ||
+        secondRule.isSubstitutionSimplified())
+      continue;
+
     bool chooseFirstRule = firstRule.getRHS().size() > secondRule.getRHS().size();
     auto subjectRule = chooseFirstRule ? firstRule : secondRule;
     auto subjectTerm = subjectRule.getRHS();

test/Generics/requirement_machine_diagnostics.swift

@@ -308,16 +308,12 @@ func sameTypeConflicts() {
 
   struct G<T> {}
 
-  // FIXME: conflict diagnosed twice
-  // expected-error@+1 2{{no type for 'T.X' can satisfy both 'T.X == G<U.Foo>' and 'T.X == Int'}}
+  // expected-error@+1{{no type for 'T.X' can satisfy both 'T.X == G<U.Foo>' and 'T.X == Int'}}
   func fail7<U: Fooable, T: Concrete>(_: U, _: T) where T.X == G<U.Foo> {}
 
-  // FIXME: conflict diagnosed twice
-  // expected-error@+2{{no type for 'T' can satisfy both 'T == Int' and 'T == G<U.Foo>'}}
   // expected-error@+1{{no type for 'T' can satisfy both 'T == G<U.Foo>' and 'T == Int'}}
   func fail8<T, U: Fooable>(_: U, _: T) where T == G<U.Foo>, T == Int {}
 
-  // FIXME: conflict diagnosed twice
-  // expected-error@+1 2{{no type for 'T' can satisfy both 'T == G<U.Foo>' and 'T == Int'}}
+  // expected-error@+1{{no type for 'T' can satisfy both 'T == G<U.Foo>' and 'T == Int'}}
   func fail9<T, U: Fooable>(_: U, _: T) where T == Int, T == G<U.Foo> {}
 }