Allow @safe(unchecked) on import decls

Author: DougGregor

include/swift/AST/DeclAttr.def

@@ -529,7 +529,7 @@ SIMPLE_DECL_ATTR(_addressableSelf, AddressableSelf,
 
 DECL_ATTR(safe, Safe,
   OnAbstractFunction | OnSubscript | OnVar | OnMacro | OnNominalType |
-  OnExtension | OnTypeAlias | UserInaccessible |
+  OnExtension | OnTypeAlias | OnImport | UserInaccessible |
   ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
   163)
 

test/Unsafe/unsafe_concurrency.swift

@@ -1,14 +1,17 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module-path %t/unsafe_swift_decls.swiftmodule %S/Inputs/unsafe_swift_decls.swift -enable-experimental-feature AllowUnsafeAttribute
+// RUN: %target-swift-frontend -emit-module-path %t/unsafe_swift_decls.swiftmodule %S/Inputs/unsafe_swift_decls.swift -enable-experimental-feature AllowUnsafeAttribute -enable-experimental-feature AllowUnsafeAttribute
 
-// RUN: %target-typecheck-verify-swift -enable-experimental-feature WarnUnsafe -enable-experimental-feature StrictConcurrency -I %t
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature WarnUnsafe -enable-experimental-feature StrictConcurrency -enable-experimental-feature AllowUnsafeAttribute -I %t
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_StrictConcurrency
 // REQUIRES: swift_feature_WarnUnsafe
+// REQUIRES: swift_feature_AllowUnsafeAttribute
 
 @preconcurrency import unsafe_swift_decls // expected-warning{{@preconcurrency import is not memory-safe because it can silently introduce data races; use '@safe(unchecked)' to assert that the code is memory-safe}}{{1-1=@safe(unchecked) }}
 
+@safe(unchecked) @preconcurrency import unsafe_swift_decls // okay
+
 class C: @unchecked Sendable {
   var counter: Int = 0
 }