NCGenerics: fix reverse condfail

I gated inverses on associated types behind a feature flag, but a
slightly older stdlib being rebuilt from source will now fail because it
 used that functionality for `_Pointer` without the feature.

So, just permit the feature if we're working with a module built from
an interface file, to avoid this reverse condfail
.

Author: kavon

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -757,8 +757,11 @@ void swift::rewriting::applyInverses(
 
     // Inverses on associated types are experimental.
     if (!allowInverseOnAssocType && canSubject->is<DependentMemberType>()) {
-      errors.push_back(RequirementError::forInvalidInverseSubject(inverse));
-      continue;
+      // Special exception: allow if we're building the stdlib.
+      if (!ctx.MainModule->isStdlibModule()) {
+        errors.push_back(RequirementError::forInvalidInverseSubject(inverse));
+        continue;
+      }
     }
 
     // Noncopyable checking support for parameter packs is not implemented yet.

test/Sema/suppressed_assoc_stdlib.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift -parse-stdlib -module-name Swift -enable-experimental-feature NoncopyableGenerics
+
+public protocol Hello {
+  associatedtype Req: ~Copyable
+}