[GSB] Flatten list of same-type requirements.

Equivalence classes stored their same-type constraints in a MapVector
keyed on the source potential archetype, which allowed traversal along the
paths of the graph. However, this capability isn't required, because we
end up walking all of the edges each time. Flatten the list of same-type
constraints to a single vector, eliminating constraint duplication between
the source and the target out-edge lists.

This cuts down on the number of same-type constraints we record by 50%,
but performance gains are limited (6% of stdlib type-checking time)
because most of the time these same-type constraints were skipped
anyway.

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -120,11 +120,8 @@ class GenericSignatureBuilder {
     llvm::MapVector<ProtocolDecl *, std::vector<Constraint<ProtocolDecl *>>>
       conformsTo;
 
-    /// Same-type constraints between each potential archetype and any other
-    /// archetype in its equivalence class.
-    llvm::MapVector<PotentialArchetype *,
-                    std::vector<Constraint<PotentialArchetype *>>>
-      sameTypeConstraints;
+    /// Same-type constraints within this equivalence class.
+    std::vector<Constraint<PotentialArchetype *>> sameTypeConstraints;
 
     /// Concrete type to which this equivalence class is equal.
     ///
@@ -1669,18 +1666,6 @@ class GenericSignatureBuilder::PotentialArchetype {
   /// potential archetype computations.
   PotentialArchetype *getArchetypeAnchor(GenericSignatureBuilder &builder);
 
-  /// Retrieve the same-type constraints.
-  ArrayRef<Constraint<PotentialArchetype *>> getSameTypeConstraints() const {
-    if (auto equivClass = getEquivalenceClassIfPresent()) {
-      auto known = equivClass->sameTypeConstraints.find(
-                                       const_cast<PotentialArchetype *>(this));
-      if (known == equivClass->sameTypeConstraints.end()) return { };
-      return known->second;
-    }
-
-    return { };
-  }
-
   /// \brief Retrieve (or create) a nested type that is the current best
   /// nested archetype anchor (locally) with the given name.
   ///