Sema: Downgrade diagnostics about inheritance from a less available type when -target-min-inlining-version min is specified.

As a concession to source compatibility for API libraries, downgrade diagnostics about inheritance from a less available type when the following conditions are met:
1. The inherited type is only potentially unavailable before the deployment target.
2. The inheriting type is `@usableFromInline`.

Resolves rdar://92747826

Author: tshortli

lib/Sema/TypeCheckAccess.cpp

@@ -1746,10 +1746,22 @@ class DeclAvailabilityChecker : public DeclVisitor<DeclAvailabilityChecker> {
   void visitNominalTypeDecl(const NominalTypeDecl *nominal) {
     checkGenericParams(nominal, nominal);
 
+    DeclAvailabilityFlags flags =
+        DeclAvailabilityFlag::AllowPotentiallyUnavailableProtocol;
+
+    // As a concession to source compatibility for API libraries, downgrade
+    // diagnostics about inheritance from a less available type when the
+    // following conditions are met:
+    // 1. The inherited type is only potentially unavailable before the
+    //    deployment target.
+    // 2. The inheriting type is `@usableFromInline`.
+    if (nominal->getAttrs().hasAttribute<UsableFromInlineAttr>())
+      flags |= DeclAvailabilityFlag::
+          WarnForPotentialUnavailabilityBeforeDeploymentTarget;
+
     llvm::for_each(nominal->getInherited(), [&](TypeLoc inherited) {
       checkType(inherited.getType(), inherited.getTypeRepr(), nominal,
-                ExportabilityReason::General,
-                DeclAvailabilityFlag::AllowPotentiallyUnavailableProtocol);
+                ExportabilityReason::General, flags);
     });
   }
 

test/attr/attr_inlinable_available.swift

@@ -1290,6 +1290,11 @@ public enum NoAvailableEnumWithClasses {
   public class InheritsBetweenTargets: BetweenTargetsClass {} // expected-error {{'BetweenTargetsClass' is only available in macOS 10.14.5 or newer; clients of 'Test' may have a lower deployment target}} expected-note 2 {{add @available attribute to enclosing class}}
   public class InheritsAtDeploymentTarget: AtDeploymentTargetClass {} // expected-error {{'AtDeploymentTargetClass' is only available in macOS 10.15 or newer; clients of 'Test' may have a lower deployment target}} expected-note 2 {{add @available attribute to enclosing class}}
   public class InheritsAfterDeploymentTarget: AfterDeploymentTargetClass {} // expected-error {{'AfterDeploymentTargetClass' is only available in macOS 11 or newer}} expected-note 2 {{add @available attribute to enclosing class}}
+  
+  // As a special case, downgrade the less available superclasses diagnostic for
+  // `@usableFromInline` classes.
+  @usableFromInline
+  class UFIInheritsBetweenTargets: BetweenTargetsClass {} // expected-warning {{'BetweenTargetsClass' is only available in macOS 10.14.5 or newer; clients of 'Test' may have a lower deployment target}} expected-note 2 {{add @available attribute to enclosing class}}
 }
 
 @_spi(Private)