[cxx-interop] Pass --gcc-toolchain to Clang driver when determining include paths

Author: egorzhdan

lib/ClangImporter/ClangImporter.cpp

@@ -902,7 +902,14 @@ getClangInvocationFileMapping(ASTContext &ctx) {
       new clang::DiagnosticOptions());
   clang::driver::Driver clangDriver(ctx.ClangImporterOpts.clangPath,
                                     triple.str(), *clangDiags);
-  llvm::opt::InputArgList clangDriverArgs;
+  // Flags passed to Swift with `-Xcc` might affect include paths.
+  unsigned unused1, unused2;
+  std::vector<const char *> clangArgs;
+  for (const auto &each : ctx.ClangImporterOpts.ExtraArgs) {
+    clangArgs.push_back(each.c_str());
+  }
+  llvm::opt::InputArgList clangDriverArgs =
+      clangDriver.getOpts().ParseArgs(clangArgs, unused1, unused2);
   // If an SDK path was explicitly passed to Swift, make sure to pass it to
   // Clang driver as well. It affects the resulting include paths.
   auto sdkPath = ctx.SearchPathOpts.getSDKPath();

test/Interop/Cxx/stdlib/libstdcxx-module-interface.swift

@@ -26,3 +26,5 @@
 // CHECK-STRING:   typealias string = std.{{__cxx11.__CxxTemplateInstNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE|__CxxTemplateInstSs}}
 // CHECK-STRING:   typealias wstring = std.{{__cxx11.__CxxTemplateInstNSt7__cxx1112basic_stringIwSt11char_traitsIwESaIwEEE|__CxxTemplateInstSbIwSt11char_traitsIwESaIwEE}}
 // CHECK-STD: }
+
+// RUN: %target-swift-ide-test -print-module -module-to-print=std -sdk / -Xcc --gcc-toolchain=/usr -source-filename=x -enable-experimental-cxx-interop -module-cache-path %t > %t/interface.swift