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/ClangImporter/ClangImporter.cpp

@@ -761,23 +761,11 @@ void importer::getNormalInvocationArguments(
   invocationArgStrs.push_back("-fapinotes-swift-version=" +
                               languageVersion.asAPINotesVersionString());
 
-  // Prefer `-sdk` paths.
-  if (!searchPathOpts.getSDKPath().empty()) {
-    llvm::SmallString<261> path{searchPathOpts.getSDKPath()};
-    llvm::sys::path::append(path, "usr", "lib", "swift", "apinotes");
-
-    invocationArgStrs.push_back("-iapinotes-modules");
-    invocationArgStrs.push_back(path.str().str());
-  }
-
-  // Fallback to "legacy" `-resource-dir` paths.
-  {
     llvm::SmallString<261> path{searchPathOpts.RuntimeResourcePath};
     llvm::sys::path::append(path, "apinotes");
 
     invocationArgStrs.push_back("-iapinotes-modules");
     invocationArgStrs.push_back(path.str().str());
-  }
 }
 
 static void

lib/Frontend/CompilerInvocation.cpp

@@ -2281,6 +2281,7 @@ 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) {
@@ -2415,6 +2416,13 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
 
   if (const Arg *A = Args.getLastArg(OPT_resource_dir))
     Opts.RuntimeResourcePath = A->getValue();
+  else if (Args.hasArg(OPT_sdk)) {
+    llvm::SmallString<128> SDKResourcePath(Opts.getSDKPath());
+    llvm::sys::path::append(
+        SDKResourcePath, "usr", "lib",
+        FrontendOpts.UseSharedResourceFolder ? "swift" : "swift_static");
+    Opts.RuntimeResourcePath = SDKResourcePath.str();
+  }
 
   Opts.SkipAllImplicitImportPaths |= Args.hasArg(OPT_nostdimport);
   Opts.SkipSDKImportPaths |= Args.hasArg(OPT_nostdlibimport);
@@ -4060,7 +4068,7 @@ 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;
   }