AST: Replace GenericSignature::createGenericEnvironment() with getGenericEnvironment()

This memoizes the result, which is fine for all callers; the only
exception is open existential types where each new open existential
now explicitly gets a unique generic environment, allocated by
calling GenericEnvironment::getIncomplete().

lib/Sema/CodeSynthesis.cpp

@@ -453,7 +453,7 @@ configureGenericDesignatedInitOverride(ASTContext &ctx,
           std::move(requirements)
         },
         nullptr);
-    genericEnv = genericSig->createGenericEnvironment();
+    genericEnv = genericSig->getGenericEnvironment();
   } else {
     genericEnv = classDecl->getGenericEnvironment();
   }