Merge pull request swiftlang#31856 from AnthonyLatsis/o-getcontextsubs

[NFC] AST: Push up a fast path in TypeBase::getContextSubstitutions

Author: AnthonyLatsis

lib/AST/Type.cpp

@@ -3973,16 +3973,16 @@ TypeBase::getContextSubstitutions(const DeclContext *dc,
     return substitutions;
   }
 
+  const auto genericSig = dc->getGenericSignatureOfContext();
+  if (!genericSig)
+    return substitutions;
+
   // Find the superclass type with the context matching that of the member.
   auto *ownerNominal = dc->getSelfNominalTypeDecl();
   if (auto *ownerClass = dyn_cast<ClassDecl>(ownerNominal))
     baseTy = baseTy->getSuperclassForDecl(ownerClass);
 
   // Gather all of the substitutions for all levels of generic arguments.
-  auto genericSig = dc->getGenericSignatureOfContext();
-  if (!genericSig)
-    return substitutions;
-
   auto params = genericSig->getGenericParams();
   unsigned n = params.size();