[Sema] Always accept @_spiOnly in a swiftinterface

Let's not print the -experimental-spi-only-imports flag in the
swiftinterface and always consider it available there. This is a more
backwards and forwards compatible way to handle swiftinterfaces
evolution.

Author: xymus

include/swift/Option/FrontendOptions.td

@@ -317,10 +317,6 @@ def enable_resilience : Flag<["-"], "enable-resilience">,
 def enable_experimental_async_top_level :
   Flag<["-"], "enable-experimental-async-top-level">,
   HelpText<"Enable experimental concurrency in top-level code">;
-
-def experimental_spi_only_imports :
-  Flag<["-"], "experimental-spi-only-imports">,
-  HelpText<"Enable use of @_spiOnly imports">;
 }
 
 // HIDDEN FLAGS
@@ -1114,4 +1110,8 @@ def enable_emit_generic_class_ro_t_list :
   Flag<["-"], "enable-emit-generic-class-ro_t-list">,
   HelpText<"Enable emission of a section with references to class_ro_t of "
            "generic class patterns">;
+
+def experimental_spi_only_imports :
+  Flag<["-"], "experimental-spi-only-imports">,
+  HelpText<"Enable use of @_spiOnly imports">;
 } // end let Flags = [FrontendOption, NoDriverOption, HelpHidden]

lib/Sema/TypeCheckAttr.cpp

@@ -3911,7 +3911,9 @@ AttributeChecker::visitImplementationOnlyAttr(ImplementationOnlyAttr *attr) {
 
 void
 AttributeChecker::visitSPIOnlyAttr(SPIOnlyAttr *attr) {
-  if (!Ctx.LangOpts.EnableSPIOnlyImports) {
+  auto *SF = D->getDeclContext()->getParentSourceFile();
+  if (!Ctx.LangOpts.EnableSPIOnlyImports &&
+      SF->Kind != SourceFileKind::Interface) {
     diagnoseAndRemoveAttr(attr, diag::spi_only_imports_not_enabled);
   }
 }

test/SPI/spi_only_import_flag_check.swift

@@ -16,7 +16,19 @@
 // RUN:   -module-name Client -emit-module-path %t/Client.swiftmodule \
 // RUN:   -experimental-spi-only-imports
 
+/// Attribute is accepted without the flag when in a swiftinterface.
+// RUN: %target-swift-typecheck-module-from-interface(%t/Client.private.swiftinterface) \
+// RUN:   -I %t -module-name Client
+
 //--- Empty.swift
 //--- Client.swift
 
 @_spiOnly import Empty // expected-error {{'@_spiOnly' requires setting the frontend flag '-experimental-spi-only-imports'}}
+
+//--- Client.private.swiftinterface
+
+// swift-interface-format-version: 1.0
+// swift-compiler-version: Swift version 5.8-dev effective-4.1.50
+// swift-module-flags: -swift-version 4 -module-name Client
+import Swift
+@_spiOnly import Empty