[ModuleInterface] Print the public keyword on imports in Swift 6 mode

If the access-level on imports proposal is accepted as written, all
imports printed in swiftinterfaces will be `public`. Whether or not we
require the explicit `public` keyword in Swift 6 mode, printing it will
have no downside. It also goes along with the mentality that
swiftinterfaces should be more explicit than implicit.

rdar://115455383

Author: xymus

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -348,6 +348,10 @@ static void printImports(raw_ostream &out,
         out << "@_spi(" << spiName << ") ";
     }
 
+    if (M->getASTContext().LangOpts.isSwiftVersionAtLeast(6)) {
+      out << "public ";
+    }
+
     out << "import ";
     if (Opts.AliasModuleNames &&
         AliasModuleNamesTargets.contains(importedModule->getName().str()))

test/ModuleInterface/imports.swift

@@ -5,6 +5,10 @@
 // RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -I %S/Inputs/imports-clang-modules/ -I %t
 // RUN: %FileCheck -implicit-check-not BAD %s < %t.swiftinterface
 
+/// Swift 6 variant.
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s %S/Inputs/imports-other.swift -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -I %S/Inputs/imports-clang-modules/ -I %t -verify -swift-version 6
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface) -I %S/Inputs/imports-clang-modules/ -I %t
+// RUN: %FileCheck -implicit-check-not BAD -check-prefix CHECK-6 %s < %t.swiftinterface
 
 @_exported import empty // expected-warning {{module 'empty' was not compiled with library evolution support; using it means binary compatibility for 'imports' can't be guaranteed}}
 @_exported import emptyButWithLibraryEvolution
@@ -29,3 +33,17 @@ import NotSoSecret2 // expected-warning {{'NotSoSecret2' inconsistently imported
 // CHECK-NEXT: {{^}}@_exported import empty{{$}}
 // CHECK-NEXT: {{^}}@_exported import emptyButWithLibraryEvolution{{$}}
 // CHECK-NOT: import
+
+// CHECK-6-NOT: import
+// CHECK-6: {{^}}public import A{{$}}
+// CHECK-6-NEXT: {{^}}public import B{{$}}
+// CHECK-6-NEXT: {{^}}public import B.B2{{$}}
+// CHECK-6-NEXT: {{^}}public import B.B3{{$}}
+// CHECK-6-NEXT: {{^}}public import C/*.c*/{{$}}
+// CHECK-6-NEXT: {{^}}public import D{{$}}
+// CHECK-6-NEXT: {{^}}public import NotSoSecret{{$}}
+// CHECK-6-NEXT: {{^}}public import NotSoSecret2{{$}}
+// CHECK-6-NEXT: {{^}}public import Swift{{$}}
+// CHECK-6-NEXT: {{^}}@_exported public import empty{{$}}
+// CHECK-6-NEXT: {{^}}@_exported public import emptyButWithLibraryEvolution{{$}}
+// CHECK-6-NOT: import