Frontend: Remove the -skip-import-in-public-interface flag.

This flag is unsafe since the compiler does not verify that the resulting
public interface will compile with the module import removed. The modern
alternative to this flag is `@_spiOnly import`. Since the flag is no longer
used by any projects it should be removed.

Resolves rdar://134351088.

Author: tshortli

include/swift/Frontend/ModuleInterfaceSupport.h

@@ -71,9 +71,6 @@ struct ModuleInterfaceOptions {
   /// Intentionally print invalid syntax into the file.
   bool DebugPrintInvalidSyntax = false;
 
-  /// A list of modules we shouldn't import in the public interfaces.
-  std::vector<std::string> ModulesToSkipInPublicInterface;
-
   /// A mode which decides whether the printed interface contains package, SPIs, or public/inlinable declarations.
   PrintOptions::InterfaceMode InterfaceContentMode = PrintOptions::InterfaceMode::Public;
   bool printPublicInterface() const {

include/swift/Option/FrontendOptions.td

@@ -1239,11 +1239,6 @@ def new_driver_path
   : Separate<["-"], "new-driver-path">, MetaVarName<"<path>">,
   HelpText<"Path of the new driver to be used">;
 
-def skip_import_in_public_interface:
-  Separate<["-"], "skip-import-in-public-interface">,
-  HelpText<"Skip the import statement corresponding to a module name "
-           "when printing the public interface.">;
-
 def disable_fine_module_tracing:
   Flag<["-"], "disable-fine-module-tracing">,
   HelpText<"Skip the emission of fine grained module tracing file.">;

lib/Frontend/CompilerInvocation.cpp

@@ -525,9 +525,6 @@ static void ParseModuleInterfaceArgs(ModuleInterfaceOptions &Opts,
       Opts.setInterfaceMode(PrintOptions::InterfaceMode::Private);
     }
   }
-  for (auto val: Args.getAllArgValues(OPT_skip_import_in_public_interface)) {
-    Opts.ModulesToSkipInPublicInterface.push_back(val);
-  }
 }
 
 /// Checks if an arg is generally allowed to be included

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -339,11 +339,6 @@ static void printImports(raw_ostream &out,
       continue;
     }
 
-    if (llvm::count(Opts.ModulesToSkipInPublicInterface,
-                    importedModule->getName().str())) {
-      continue;
-    }
-
     llvm::SmallSetVector<Identifier, 4> spis;
     M->lookupImportedSPIGroups(importedModule, spis);
 

lib/FrontendTool/FrontendTool.cpp

@@ -871,7 +871,6 @@ static bool emitAnyWholeModulePostTypeCheckSupplementaryOutputs(
     // Copy the settings from the module interface to add SPI printing.
     ModuleInterfaceOptions privOpts = Invocation.getModuleInterfaceOptions();
     privOpts.setInterfaceMode(PrintOptions::InterfaceMode::Private);
-    privOpts.ModulesToSkipInPublicInterface.clear();
 
     hadAnyError |= printModuleInterfaceIfNeeded(
         Instance.getOutputBackend(),
@@ -884,7 +883,6 @@ static bool emitAnyWholeModulePostTypeCheckSupplementaryOutputs(
     // Copy the settings from the module interface to add package decl printing.
     ModuleInterfaceOptions pkgOpts = Invocation.getModuleInterfaceOptions();
     pkgOpts.setInterfaceMode(PrintOptions::InterfaceMode::Package);
-    pkgOpts.ModulesToSkipInPublicInterface.clear();
 
     hadAnyError |= printModuleInterfaceIfNeeded(
         Instance.getOutputBackend(),