Sema: test @_spiOnly with warnings about superfluously public imports

The logic reporting superfluously public imports treats @_spiOnly
imports as any public import. As such @_spiOnly shouldn't interfere with
the diagnosics. Let's make sure we test the current behavior.

In the future, we may want to extend the existing logic to report bare
`public imports` that could be `@_spiOnly public import` if referred
from SPI but not API.

Author: xymus

test/Sema/superfluously-public-imports.swift

@@ -21,10 +21,12 @@
 // RUN: %target-swift-frontend -emit-module %t/ImportNotUseFromAPI.swift -o %t -I %t
 // RUN: %target-swift-frontend -emit-module %t/ImportUsedInPackage.swift -o %t -I %t
 // RUN: %target-swift-frontend -emit-module %t/ExportedUnused.swift -o %t -I %t
+// RUN: %target-swift-frontend -emit-module %t/SPIOnlyUsedInSPI.swift -o %t -I %t
 
 /// Check diagnostics.
 // RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
-// RUN:   -package-name pkg -Rmodule-api-import -swift-version 6 -verify
+// RUN:   -package-name pkg -Rmodule-api-import -swift-version 6 -verify \
+// RUN:   -experimental-spi-only-imports
 // RUN: %target-swift-frontend -typecheck %t/ClientOfClangModules.swift -I %t \
 // RUN:   -package-name pkg -Rmodule-api-import -swift-version 6 -verify
 // RUN: %target-swift-frontend -typecheck %t/Client_Swift5.swift -I %t \
@@ -105,6 +107,9 @@ public func packageFunc() -> PackageType { return PackageType() }
 
 //--- ExportedUnused.swift
 
+//--- SPIOnlyUsedInSPI.swift
+public struct ToUseFromSPI {}
+
 //--- Client_Swift5.swift
 /// No diagnostics should be raised on the implicit access level.
 import UnusedImport // expected-error {{ambiguous implicit access level for import of 'UnusedImport'; it is imported as 'public' elsewhere}}
@@ -137,6 +142,7 @@ public import ImportNotUseFromAPI // expected-warning {{public import of 'Import
 public import ImportUsedInPackage // expected-warning {{public import of 'ImportUsedInPackage' was not used in public declarations or inlinable code}} {{1-7=package}}
 
 @_exported public import ExportedUnused
+@_spiOnly public import SPIOnlyUsedInSPI
 
 public func useInSignature(_ a: TypeUsedInSignature) {} // expected-remark {{struct 'TypeUsedInSignature' is imported via 'DepUsedInSignature'}}
 public func exportedTypeUseInSignature(_ a: ExportedType) {} // expected-remark {{struct 'ExportedType' is imported via 'Exporter', which reexports definition from 'Exportee'}}
@@ -219,6 +225,9 @@ func implicitlyInternalFunc(a: NotAnAPIType = notAnAPIFunc()) {}
 // For package decls we only remark on types used in signatures, not for inlinable code.
 package func packageFunc(a: PackageType = packageFunc()) {} // expected-remark {{struct 'PackageType' is imported via 'ImportUsedInPackage'}}
 
+@_spi(X)
+public func spiFunc(a: ToUseFromSPI) {} // expected-remark {{struct 'ToUseFromSPI' is imported via 'SPIOnlyUsedInSPI'}}
+
 /// Tests for imports of clang modules.
 //--- module.modulemap
 module ClangSimpleUnused {