Remove internal type from __Sealed attribute in implicit_context.hhi

D74667088 marked MemoSensitiveImplicitContext as __Sealed, with only a
single class permitted to directly extend it. This class is however
internal to Meta, causing typechecking errors in OSS Hack.

As a fix, mark the attribute with `// @oss-disable` to remove
it during the code export process.

NOTE: If I'm understanding the docs[1] for `@oss-disable` / `@oss-enable` correctly,
this should be the correct way to make this change, which should then
transform into `// @oss-disable FBMemoAgnosticImplicitContext::class`
once this patch is merged and exported. Let me know if this assumption
is incorrect.

[1] https://github.com/facebook/buck2/blob/239ab927a5be7dee3035141d29e2e9c91e8ea771/HACKING.md?plain=1#L168

Author: mszabo-wikia

hphp/hack/hhi/implicit_context.hhi

@@ -48,7 +48,8 @@ namespace HH {
     )[this::CRun, ctx $f]: Tout;
   }
 
-  <<__Sealed(FBMemoSensitiveImplicitContext::class)>>
+  // Avoid referencing an internal class in OSS.
+  <<__Sealed(FBMemoAgnosticImplicitContext::class)>> // @oss-disable
   abstract class MemoSensitiveImplicitContext extends ImplicitContextBase {
     abstract const type TData as IPureMemoizeParam;