Make swiftinterfaces propagate #file behavior

Add -experimental-enable-concise-pound-file to the list of flags preserved by module interfaces, so that when we rebuild an interface, it comes out the same way as the original file.

Author: beccadax

include/swift/Option/Options.td

@@ -516,7 +516,7 @@ def enable_experimental_additive_arithmetic_derivation :
 
 def enable_experimental_concise_pound_file : Flag<["-"],
     "enable-experimental-concise-pound-file">,
-  Flags<[FrontendOption]>,
+  Flags<[FrontendOption, ModuleInterfaceOption]>,
   HelpText<"Enable experimental concise '#file' identifier">;
 
 // Diagnostic control options

test/SILGen/magic_identifier_file.swift

@@ -12,6 +12,15 @@
 // Test in "Swift 6" mode:
 // RUN: %target-swift-emit-silgen -I %t -enable-experimental-concise-pound-file -module-name Foo %/s | %FileCheck --check-prefixes=BOTH,CONCISE %s
 
+// Remove compiled modules so we test against interfaces:
+// RUN: rm %t/MagicIdentifierFileSwift5.swiftmodule %t/MagicIdentifierFileSwift6.swiftmodule
+
+// Test in Swift 5 mode:
+// RUN: %target-swift-emit-silgen -I %t -module-name Foo %/s | %FileCheck --check-prefixes=BOTH,ABSOLUTE %s
+
+// Test in "Swift 6" mode:
+// RUN: %target-swift-emit-silgen -I %t -enable-experimental-concise-pound-file -module-name Foo %/s | %FileCheck --check-prefixes=BOTH,CONCISE %s
+
 import MagicIdentifierFileSwift5
 import MagicIdentifierFileSwift6