Option to disable printing package-name in public or private interface.

Having package-name flag in non-package interfaces causes them to be built as if
belonging to a package, which causes an issue for a loading client outside of the
package as follows.

For example, when building X that depends on A with the following dependency chain:
  X --> A --> B --(package-only)--> C

1. X itself is not in the same package as A, B, and C.
2. When dependency scanning X, and opening up B, because the scan target is in a
   different package domain, the scanner decides that B's package-only dependency
   on C is to be ignored.
3. When then finally building A itself, it will load its dependencies, but because
   the .private.swiftinterface of A still specifies -package-name, when it loads
   B, it will then examine its dependencies and deem that this package-only dependency
   on C is required.

Because (2) and (3) disagree, we get an error now when building the private A textual interface.

rdar://130701866

include/swift/Frontend/ModuleInterfaceSupport.h

@@ -62,6 +62,10 @@ struct ModuleInterfaceOptions {
   /// Print imports that are missing from the source and used in API.
   bool PrintMissingImports = true;
 
+  /// If true, package-name flag is not printed in either public or private
+  /// interface file.
+  bool DisablePackageNameForNonPackageInterface = false;
+
   /// Intentionally print invalid syntax into the file.
   bool DebugPrintInvalidSyntax = false;