AST: Fix a bug in ReplaceOpaqueTypesWithUnderlyingTypes

There was a logic error in canSubstituteTypeInto(). An existing test
(test/IRGen/opaque_result_type_private_underlying.swift) triggered
this issue with a change I'm working on. Upon further inspection
I noticed the problem can already be triggered in the same test if
we build with -O.

The problem was that we were too agressive with substituting away
opaque types away in the body of an inlinable function in a
non-resilient build. While opaque types are not resilient in this
case, we still need to keep them around if the underlying type is
insufficiently visible.

Author: slavapestov

lib/AST/Type.cpp

@@ -3150,7 +3150,7 @@ static bool canSubstituteTypeInto(Type ty, const DeclContext *dc,
   case OpaqueSubstitutionKind::SubstituteNonResilientModule:
     // Can't access types that are not public from a different module.
     if (dc->getParentModule() == typeDecl->getDeclContext()->getParentModule())
-      return true;
+      return typeDecl->getEffectiveAccess() > AccessLevel::FilePrivate;
 
     return typeDecl->getEffectiveAccess() > AccessLevel::Internal;
   }

test/SILGen/Inputs/opaque_result_type_inlinable_other.swift

@@ -0,0 +1,5 @@
+fileprivate struct PrivateS {}
+
+public func callee() -> some Any {
+  return PrivateS()
+}

test/SILGen/opaque_result_type_inlinable.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-frontend -disable-availability-checking -emit-silgen -primary-file %s -primary-file %S/Inputs/opaque_result_type_inlinable_other.swift | %FileCheck %s
+// RUN: %target-swift-frontend -disable-availability-checking -emit-silgen -primary-file %s %S/Inputs/opaque_result_type_inlinable_other.swift | %FileCheck %s
+// RUN: %target-swift-frontend -disable-availability-checking -emit-silgen %s %S/Inputs/opaque_result_type_inlinable_other.swift | %FileCheck %s
+
+// CHECK-LABEL: sil [serialized] [ossa] @$s28opaque_result_type_inlinable6callerQryF : $@convention(thin) () -> @out @_opaqueReturnTypeOf("$s28opaque_result_type_inlinable6callerQryF", 0) __ {
+// CHECK: bb0(%0 : $*@_opaqueReturnTypeOf("$s28opaque_result_type_inlinable6calleeQryF", 0) __):
+// CHECK: function_ref @$s28opaque_result_type_inlinable6calleeQryF : $@convention(thin) () -> @out @_opaqueReturnTypeOf("$s28opaque_result_type_inlinable6calleeQryF", 0) __
+
+@inlinable public func caller() -> some Any {
+  return callee()
+}