[ModuleInterface] Print alias for module names in swiftinterface files

Ambiguities are introduced in generated swiftinterfaces when a type
shares a name with a module (i.e. XCTest). This workaround uses the
module-alias feature to avoid these ambiguities. Writing module
references with a distinguishable prefix should allow normal
type-checking to avoid the usual ambiguities.

We should still aim for a proper fully-qualified named syntax, but this
may help in the mean time.

rdar://101969500

include/swift/Frontend/ModuleInterfaceSupport.h

@@ -33,6 +33,10 @@ struct ModuleInterfaceOptions {
   /// interface, or should we fully-qualify them?
   bool PreserveTypesAsWritten = false;
 
+  /// Use aliases when printing references to modules to avoid ambiguities
+  /// with types sharing a name with a module.
+  bool AliasModuleNames = false;
+
   /// See \ref FrontendOptions.PrintFullConvention.
   /// [TODO: Clang-type-plumbing] This check should go away.
   bool PrintFullConvention = false;