[Driver][Frontend] Add a -nostdlibimport argument

Add a -nostdlibimport (analagous to clang's -nostdlibinc) to remove the SDK paths from the import search paths, but leave the toolchain paths.

rdar://139322299

Author: ian-twilightcoder

include/swift/AST/SearchPathOptions.h

@@ -507,6 +507,9 @@ class SearchPathOptions {
   /// Don't look in for compiler-provided modules.
   bool SkipRuntimeLibraryImportPaths = false;
 
+  /// Don't include SDK paths in the RuntimeLibraryImportPaths
+  bool ExcludeSDKPathsFromRuntimeLibraryImportPaths = false;
+
   /// Scanner Prefix Mapper.
   std::vector<std::string> ScannerPrefixMapper;
 

include/swift/Option/Options.td

@@ -359,6 +359,8 @@ def incremental : Flag<["-"], "incremental">,
   HelpText<"Perform an incremental build if possible">;
 
 def nostdimport : Flag<["-"], "nostdimport">, Flags<[FrontendOption]>,
+  HelpText<"Don't search the standard library or toolchain import paths for modules">;
+def nostdlibimport : Flag<["-"], "nostdlibimport">, Flags<[FrontendOption]>,
   HelpText<"Don't search the standard library import path for modules">;
 
 def output_file_map : Separate<["-"], "output-file-map">,

lib/Driver/ToolChains.cpp

@@ -296,6 +296,7 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_package_name);
   inputArgs.AddLastArg(arguments, options::OPT_export_as);
   inputArgs.AddLastArg(arguments, options::OPT_nostdimport);
+  inputArgs.AddLastArg(arguments, options::OPT_nostdlibimport);
   inputArgs.AddLastArg(arguments, options::OPT_parse_stdlib);
   inputArgs.AddLastArg(arguments, options::OPT_resource_dir);
   inputArgs.AddLastArg(arguments, options::OPT_solver_memory_threshold);

lib/Frontend/CompilerInvocation.cpp

@@ -283,7 +283,7 @@ static void updateRuntimeLibraryPaths(SearchPathOptions &SearchPathOpts,
     RuntimeLibraryImportPaths.push_back(std::string(LibPath.str()));
   }
 
-  if (!SearchPathOpts.getSDKPath().empty()) {
+  if (!SearchPathOpts.ExcludeSDKPathsFromRuntimeLibraryImportPaths && !SearchPathOpts.getSDKPath().empty()) {
     const char *swiftDir = FrontendOpts.UseSharedResourceFolder
       ? "swift" : "swift_static";
 
@@ -2261,6 +2261,7 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
     Opts.RuntimeResourcePath = A->getValue();
 
   Opts.SkipRuntimeLibraryImportPaths |= Args.hasArg(OPT_nostdimport);
+  Opts.ExcludeSDKPathsFromRuntimeLibraryImportPaths |= Args.hasArg(OPT_nostdlibimport);
 
   Opts.DisableModulesValidateSystemDependencies |=
       Args.hasArg(OPT_disable_modules_validate_system_headers);