Frontend: add module interface flag to specify allowable clients

This new flag should allow module authors to specify allowable client list. This list is printed
in the textual module interface. Diagnostics support can come later.

Author: nkcsgexi

include/swift/Option/Options.td

@@ -1340,6 +1340,10 @@ def user_module_version : Separate<["-"], "user-module-version">,
   HelpText<"Module version specified from Swift module authors">,
   MetaVarName<"<vers>">;
 
+def allowable_client : Separate<["-"], "allowable-client">,
+  Flags<[FrontendOption, ModuleInterfaceOptionIgnorable, NewDriverOnlyOption]>,
+  HelpText<"Module names that are allowed to import this module">,
+  MetaVarName<"<vers>">;
 // VFS
 
 def vfsoverlay : JoinedOrSeparate<["-"], "vfsoverlay">,

test/ModuleInterface/allowable-client.swift

@@ -0,0 +1,10 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/textual)
+// RUN: %empty-directory(%t/binary)
+
+// RUN: %target-swift-frontend -emit-module %s -module-name Foo -swift-version 5 -disable-implicit-concurrency-module-import -allowable-client FooFriend1 -allowable-client FooFriend2 -allowable-client FooFriend3 -emit-module-interface-path %t/textual/Foo.swiftinterface -enable-library-evolution -emit-module-path %t/binary/Foo.swiftmodule
+
+// RUN: %FileCheck %s --check-prefix=INTERFACE-FLAG < %t/textual/Foo.swiftinterface
+
+// INTERFACE-FLAG: swift-module-flags-ignorable:
+// INTERFACE-FLAG: -allowable-client FooFriend1 -allowable-client FooFriend2 -allowable-client FooFriend3