Have the frontend and new swift-driver look in an external -sdk for non-Darwin platform runtime libraries and modules too

as done originally in #25990 with the legacy C++ Driver, but since lost in the new swift-driver

Author: finagolfin

lib/Frontend/CompilerInvocation.cpp

@@ -2166,6 +2166,7 @@ static bool validateSwiftModuleFileArgumentAndAdd(const std::string &swiftModule
 
 static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
                                 DiagnosticEngine &Diags,
+                                const llvm::Triple &Triple,
                                 const CASOptions &CASOpts,
                                 const FrontendOptions &FrontendOpts,
                                 StringRef workingDirectory) {
@@ -2300,6 +2301,18 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
 
   if (const Arg *A = Args.getLastArg(OPT_resource_dir))
     Opts.RuntimeResourcePath = A->getValue();
+  else if (!Triple.isOSDarwin() && Args.hasArg(OPT_sdk)) {
+    llvm::SmallString<128> SDKResourcePath(Opts.getSDKPath());
+    llvm::sys::path::append(
+        SDKResourcePath, "usr", "lib",
+        FrontendOpts.UseSharedResourceFolder ? "swift" : "swift_static",
+        getPlatformNameForTriple(Triple));
+    // Check for eg <sdkRoot>/usr/lib/swift/linux/
+    if (llvm::sys::fs::exists(SDKResourcePath)) {
+      llvm::sys::path::remove_filename(SDKResourcePath); // Remove <os> name
+      Opts.RuntimeResourcePath = SDKResourcePath.str();
+    }
+  }
 
   Opts.SkipRuntimeLibraryImportPaths |= Args.hasArg(OPT_nostdimport);
   Opts.ExcludeSDKPathsFromRuntimeLibraryImportPaths |= Args.hasArg(OPT_nostdlibimport);
@@ -3908,7 +3921,7 @@ bool CompilerInvocation::parseArgs(
 
   ParseSymbolGraphArgs(SymbolGraphOpts, ParsedArgs, Diags, LangOpts);
 
-  if (ParseSearchPathArgs(SearchPathOpts, ParsedArgs, Diags,
+  if (ParseSearchPathArgs(SearchPathOpts, ParsedArgs, Diags, LangOpts.Target,
                           CASOpts, FrontendOpts, workingDirectory)) {
     return true;
   }