Make OpaqueSubstitutionKind an enum class. No functionality change.

Also fix a comment typo while I'm here.

Author: jrose-apple

include/swift/AST/Types.h

@@ -4876,7 +4876,7 @@ class OpaqueTypeArchetypeType final : public ArchetypeType,
 BEGIN_CAN_TYPE_WRAPPER(OpaqueTypeArchetypeType, ArchetypeType)
 END_CAN_TYPE_WRAPPER(OpaqueTypeArchetypeType, ArchetypeType)
 
-enum OpaqueSubstitutionKind {
+enum class OpaqueSubstitutionKind {
   // Don't substitute the opaque type for the underlying type.
   DontSubstitute,
   // Substitute without looking at the type and context.
@@ -4885,7 +4885,7 @@ enum OpaqueSubstitutionKind {
   AlwaysSubstitute,
   // Substitute in the same module into a maximal resilient context.
   // Can be done if the underlying type is accessible from the context we
-  // substitute into. Private types cannot be accesses from a different TU.
+  // substitute into. Private types cannot be accessed from a different TU.
   SubstituteSameModuleMaximalResilience,
   // Substitute in a different module from the opaque definining decl. Can only
   // be done if the underlying type is public.

lib/AST/Type.cpp

@@ -2660,21 +2660,21 @@ bool canSubstituteTypeInto(Type ty, DeclContext *dc,
     return true;
 
   switch (kind) {
-  case DontSubstitute:
+  case OpaqueSubstitutionKind::DontSubstitute:
     return false;
 
-  case AlwaysSubstitute:
+  case OpaqueSubstitutionKind::AlwaysSubstitute:
     return true;
 
-  case SubstituteSameModuleMaximalResilience:
+  case OpaqueSubstitutionKind::SubstituteSameModuleMaximalResilience:
     // In the same file any visibility is okay.
     if (!dc->isModuleContext() &&
         nominal->getDeclContext()->getParentSourceFile() ==
         dc->getParentSourceFile())
       return true;
     return nominal->getEffectiveAccess() > AccessLevel::FilePrivate;
 
-  case SubstituteNonResilientModule:
+  case OpaqueSubstitutionKind::SubstituteNonResilientModule:
     // Can't access types that are not public from a different module.
     return nominal->getEffectiveAccess() > AccessLevel::Internal;
   }

lib/SILOptimizer/Transforms/SpecializeOpaqueArchetypes.cpp

@@ -508,10 +508,12 @@ class OpaqueArchetypeSpecializer : public SILFunctionTransform {
           if (!dc)
             dc = context->getModule().getSwiftModule();
           auto opaque = opaqueTy->getDecl();
-          return ReplaceOpaqueTypesWithUnderlyingTypes::
+          OpaqueSubstitutionKind subKind =
+              ReplaceOpaqueTypesWithUnderlyingTypes::
               shouldPerformSubstitution(opaque,
                                         context->getModule().getSwiftModule(),
                                         context->getResilienceExpansion());
+          return subKind != OpaqueSubstitutionKind::DontSubstitute;
         }
         return false;
       });