NCGenerics: claim to have Escapable runtime info.

We are suppose to be able to query whether a type is Escapable or not at
runtime, in order to handle plain extensions of conditionally-Escapable
types, which are implicitly like this:

```
struct S<T: ~Escapable>: ~Escapable {}
extension S: Escapable where T: Escapable {}

extension S: P where T: Escapable {} // <- requires runtime information
```

While that's not yet implemented, it eventually will be once
NonescapableTypes matures.

Author: kavon

lib/Sema/TypeCheckProtocol.cpp

@@ -2208,7 +2208,8 @@ static bool hasAdditionalSemanticChecks(ProtocolDecl *proto) {
 /// runtime for an arbitrary type.
 static bool hasRuntimeConformanceInfo(ProtocolDecl *proto) {
   return !proto->isMarkerProtocol()
-      || proto->isSpecificProtocol(KnownProtocolKind::Copyable);
+      || proto->isSpecificProtocol(KnownProtocolKind::Copyable)
+      || proto->isSpecificProtocol(KnownProtocolKind::Escapable);
 }
 
 static void ensureRequirementsAreSatisfied(ASTContext &ctx,