This code injects an LLVM modulemap for glibc and libstdc++ by overriding
specific VFS paths. In order to do that, it needs to know the actual locations
of glibc and libstdc++, but it only searches `-sysroot` and fails. Here we
patch it to also consider `-idirafter` and `-isystem` as added by cc-wrapper.

--- a/lib/ClangImporter/ClangIncludePaths.cpp
+++ b/lib/ClangImporter/ClangIncludePaths.cpp
@@ -142,6 +142,7 @@
 /// \return a path without dots (`../`, './').
 static llvm::Optional<Path> findFirstIncludeDir(
     const llvm::opt::InputArgList &args,
+    const llvm::opt::ArgList &DriverArgs,
     const ArrayRef<const char *> expectedFileNames,
     const llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> &vfs) {
   // C++ stdlib paths are added as `-internal-isystem`.
@@ -151,6 +152,14 @@
   llvm::append_range(includeDirs,
                      args.getAllArgValues(
                          clang::driver::options::OPT_internal_externc_isystem));
+  // Nix adds the C stdlib include path using `-idirafter`.
+  llvm::append_range(includeDirs,
+                     DriverArgs.getAllArgValues(
+                         clang::driver::options::OPT_idirafter));
+  // Nix adds the C++ stdlib include path using `-cxx-isystem`.
+  llvm::append_range(includeDirs,
+                     DriverArgs.getAllArgValues(
+                         clang::driver::options::OPT_cxx_isystem));
 
   for (const auto &includeDir : includeDirs) {
     Path dir(includeDir);
@@ -218,7 +227,7 @@
   // modulemap are present.
   Path glibcDir;
   if (auto dir = findFirstIncludeDir(
-          parsedIncludeArgs, {"inttypes.h", "unistd.h", "stdint.h"}, vfs)) {
+          parsedIncludeArgs, clangDriverArgs, {"inttypes.h", "unistd.h", "stdint.h"}, vfs)) {
     glibcDir = dir.value();
   } else {
     ctx.Diags.diagnose(SourceLoc(), diag::glibc_not_found, triple.str());
@@ -276,7 +285,7 @@
   auto parsedStdlibArgs = parseClangDriverArgs(clangDriver, stdlibArgStrings);
 
   Path cxxStdlibDir;
-  if (auto dir = findFirstIncludeDir(parsedStdlibArgs,
+  if (auto dir = findFirstIncludeDir(parsedStdlibArgs, clangDriverArgs,
                                      {"cstdlib", "string", "vector"}, vfs)) {
     cxxStdlibDir = dir.value();
   } else {