[SubstitutionMap] Canonicalize non-substitutable replacement types.

When we compute a replacement type for a non-substitutable generic
parameter, canonicalize the type when our generic signature is
canonical. This ensures that the canonical substitution map will
have canonical types for all of the replacement types… including those
for non-substitutable generic parameters that aren’t part of the
FoldingSet profile.

Author: DougGregor

lib/AST/SubstitutionMap.cpp

@@ -275,6 +275,11 @@ Type SubstitutionMap::lookupSubstitution(CanSubstitutableType type) const {
 
     // Substitute into the replacement type.
     replacementType = concreteType.subst(*this);
+
+    // If the generic signature is canonical, canonicalize the replacement type.
+    if (getGenericSignature()->isCanonical())
+      replacementType = replacementType->getCanonicalType();
+
     return replacementType;
   }
 
@@ -290,6 +295,11 @@ Type SubstitutionMap::lookupSubstitution(CanSubstitutableType type) const {
   replacementType = ErrorType::get(type);
 
   replacementType = lookupSubstitution(cast<SubstitutableType>(canonicalType));
+
+  // If the generic signature is canonical, canonicalize the replacement type.
+  if (getGenericSignature()->isCanonical())
+    replacementType = replacementType->getCanonicalType();
+
   return replacementType;
 }