Further improve AvailabilityWalker changes

Access context more quickly; document each flag’s purpose; perform a faster test before a slower one.

lib/Sema/TypeCheckAvailability.cpp

@@ -2311,7 +2311,7 @@ class AvailabilityWalker : public ASTWalker {
            init->isImplicit() &&
            init->getParameters()->size() == 1 &&
            init->getParameters()->get(0)->getArgumentName() ==
-                   DC->getASTContext().Id_rawValue;
+                   TC.Context.Id_rawValue;
   }
 
 public:
@@ -2604,11 +2604,11 @@ AvailabilityWalker::diagAvailability(const ValueDecl *D, SourceRange R,
   if (!isAccessorWithDeprecatedStorage)
     TC.diagnoseIfDeprecated(R, DC, D, call);
 
-  if (Flags.contains(DeclAvailabilityFlag::AllowPotentiallyUnavailableProtocol)
+  if (Flags.contains(DeclAvailabilityFlag::AllowPotentiallyUnavailable))
-        && isa<ProtocolDecl>(D))
     return false;
 
-  if (Flags.contains(DeclAvailabilityFlag::AllowPotentiallyUnavailable))
+  if (Flags.contains(DeclAvailabilityFlag::AllowPotentiallyUnavailableProtocol)
+        && isa<ProtocolDecl>(D))
     return false;
 
   // Diagnose (and possibly signal) for potential unavailability

lib/Sema/TypeCheckAvailability.h

@@ -35,9 +35,21 @@ void diagAvailability(TypeChecker &TC, const Expr *E,
                       DeclContext *DC);
 
 enum class DeclAvailabilityFlag : uint8_t {
+  /// Do not diagnose uses of protocols in versions before they were introduced.
+  /// Used when type-checking protocol conformances, since conforming to a
+  /// protocol that doesn't exist yet is allowed.
   AllowPotentiallyUnavailableProtocol = 1 << 0,
+
+  /// Diagnose uses of declarations in versions before they were introduced, but
+  /// do not return true to indicate that a diagnostic was emitted.
   ContinueOnPotentialUnavailability = 1 << 1,
+
+  /// If a diagnostic must be emitted, use a variant indicating that the usage
+  /// is inout and both the getter and setter must be available.
   ForInout = 1 << 2,
+
+  /// Do not diagnose uses of declarations in versions before they were
+  /// introduced. Used to work around availability-checker bugs.
   AllowPotentiallyUnavailable = 1 << 3,
 };
 using DeclAvailabilityFlags = OptionSet<DeclAvailabilityFlag>;