hurd: find c++ headers

This should be factorized with Linux.cpp and the GNU/kFreeBSD case.

Index: llvm-toolchain-snapshot_11~++20200207025749+c29003813ab/clang/lib/Driver/ToolChains/Hurd.cpp
===================================================================
--- llvm-toolchain-snapshot_11~++20200207025749+c29003813ab.orig/clang/lib/Driver/ToolChains/Hurd.cpp
+++ llvm-toolchain-snapshot_11~++20200207025749+c29003813ab/clang/lib/Driver/ToolChains/Hurd.cpp
@@ -89,8 +89,40 @@ Hurd::Hurd(const Driver &D, const llvm::
   addPathIfExists(D, SysRoot + "/usr/lib/" + MultiarchTriple, Paths);
   addPathIfExists(D, SysRoot + "/usr/lib/../" + OSLibDir, Paths);
 
-  // If we are currently running Clang inside of the requested system root, add
-  // its parent library path to those searched.
+  // Try walking via the GCC triple path in case of biarch or multiarch GCC
+  // installations with strange symlinks.
+  if (GCCInstallation.isValid()) {
+    addPathIfExists(D,
+                    SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
+                        "/../../" + OSLibDir,
+                    Paths);
+
+    // Add the 'other' biarch variant path
+    Multilib BiarchSibling;
+    if (GCCInstallation.getBiarchSibling(BiarchSibling)) {
+      addPathIfExists(D, GCCInstallation.getInstallPath() +
+                             BiarchSibling.gccSuffix(),
+                      Paths);
+    }
+
+    // See comments above on the multilib variant for details of why this is
+    // included even from outside the sysroot.
+    const std::string &LibPath = GCCInstallation.getParentLibPath();
+    const llvm::Triple &GCCTriple = GCCInstallation.getTriple();
+    const Multilib &Multilib = GCCInstallation.getMultilib();
+    addPathIfExists(D, LibPath + "/../" + GCCTriple.str() + "/lib" +
+                           Multilib.osSuffix(),
+                    Paths);
+
+    // See comments above on the multilib variant for details of why this is
+    // only included from within the sysroot.
+    if (StringRef(LibPath).startswith(SysRoot))
+      addPathIfExists(D, LibPath, Paths);
+  }
+
+  // Similar to the logic for GCC above, if we are currently running Clang
+  // inside of the requested system root, add its parent library path to those
+  // searched.
   // FIXME: It's not clear whether we should use the driver's installed
   // directory ('Dir' below) or the ResourceDir.
   if (StringRef(D.Dir).startswith(SysRoot))
@@ -157,6 +189,17 @@ void Hurd::AddClangSystemIncludeArgs(con
 
   // Lacking those, try to detect the correct set of system includes for the
   // target triple.
+
+  // Add include directories specific to the selected multilib set and multilib.
+  if (GCCInstallation.isValid()) {
+    const auto &Callback = Multilibs.includeDirsCallback();
+    if (Callback) {
+      for (const auto &Path : Callback(GCCInstallation.getMultilib()))
+        addExternCSystemIncludeIfExists(
+            DriverArgs, CC1Args, GCCInstallation.getInstallPath() + Path);
+    }
+  }
+
   if (getTriple().getArch() == llvm::Triple::x86) {
     std::string Path = SysRoot + "/usr/include/i386-gnu";
     if (D.getVFS().exists(Path))