GSB: Use a SmallSetVector to store the explicit requirements

We can literally have duplicate requirements (same source and RHS)
when abstract signatures are built from scratch. They're not
flagged as "redundant" since there's no way to distinguish them
in the requirement graph, so just filter them out entirely.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -698,7 +698,7 @@ struct GenericSignatureBuilder::Implementation {
   bool HadAnyError = false;
 
   /// All explicit non-same type requirements that were added to the builder.
-  SmallVector<ExplicitRequirement, 2> ExplicitRequirements;
+  SmallSetVector<ExplicitRequirement, 2> ExplicitRequirements;
 
   /// All explicit same-type requirements that were added to the builder.
   SmallVector<Requirement, 2> ExplicitSameTypeRequirements;
@@ -4507,8 +4507,9 @@ ConstraintResult GenericSignatureBuilder::addConformanceRequirement(
   auto resolvedSource = source.getSource(*this, type);
 
   if (!resolvedSource->isDerivedRequirement()) {
-    Impl->ExplicitRequirements.emplace_back(RequirementKind::Conformance,
-                                            resolvedSource, proto);
+    Impl->ExplicitRequirements.insert(
+        ExplicitRequirement(RequirementKind::Conformance,
+                            resolvedSource, proto));
   }
 
   // Add the conformance requirement, bailing out earlier if we've already
@@ -4529,8 +4530,9 @@ ConstraintResult GenericSignatureBuilder::addLayoutRequirementDirect(
   auto resolvedSource = source.getSource(*this, type);
 
   if (!resolvedSource->isDerivedRequirement()) {
-    Impl->ExplicitRequirements.emplace_back(RequirementKind::Layout,
-                                            resolvedSource, layout);
+    Impl->ExplicitRequirements.insert(
+        ExplicitRequirement(RequirementKind::Layout,
+                            resolvedSource, layout));
   }
 
   auto equivClass = type.getEquivalenceClass(*this);
@@ -4670,8 +4672,9 @@ ConstraintResult GenericSignatureBuilder::addSuperclassRequirementDirect(
   auto resolvedSource = source.getSource(*this, type);
 
   if (!resolvedSource->isDerivedRequirement()) {
-    Impl->ExplicitRequirements.emplace_back(RequirementKind::Superclass,
-                                            resolvedSource, superclass);
+    Impl->ExplicitRequirements.insert(
+        ExplicitRequirement(RequirementKind::Superclass,
+                            resolvedSource, superclass));
   }
 
   // Record the constraint.