Sema: Disallow protocols from refining less available protocols

Concrete types can conform to unavailable protocols because
the witness table for the conformance is not required for use
with the concrete type itself.

However, protocols cannot have unavailable base protocols.
I believe this was an oversight of the original implementation
here.

Author: slavapestov

lib/Sema/TypeCheckAccess.cpp

@@ -2232,7 +2232,7 @@ class DeclAvailabilityChecker : public DeclVisitor<DeclAvailabilityChecker> {
     llvm::for_each(proto->getInherited(),
                   [&](TypeLoc requirement) {
       checkType(requirement.getType(), requirement.getTypeRepr(), proto,
-                /*allowUnavailableProtocol=*/true);
+                /*allowUnavailableProtocol=*/false);
     });
 
     if (proto->getTrailingWhereClause()) {

test/Sema/availability_versions.swift

@@ -883,7 +883,7 @@ protocol ProtocolAvailableOn10_51 {
 }
 
 @available(OSX, introduced: 10.9)
-protocol ProtocolAvailableOn10_9InheritingFromProtocolAvailableOn10_51 : ProtocolAvailableOn10_51 {
+protocol ProtocolAvailableOn10_9InheritingFromProtocolAvailableOn10_51 : ProtocolAvailableOn10_51 { // expected-error {{'ProtocolAvailableOn10_51' is only available in macOS 10.51 or newer}}
 }
 
 @available(OSX, introduced: 10.51)