Use the Nix include dirs and gcc runtime dir, when no sysroot is configured.

--- clang/lib/Driver/ToolChains/Linux.cpp	2018-10-05 18:01:15.731109551 +0200
+++ clang/lib/Driver/ToolChains/Linux.cpp	2018-10-05 18:00:27.959509924 +0200
@@ -665,7 +665,7 @@
 
   // Check for configure-time C include directories.
   StringRef CIncludeDirs(C_INCLUDE_DIRS);
-  if (CIncludeDirs != "") {
+  if (CIncludeDirs != "" && (SysRoot.empty() || SysRoot == "/")) {
     SmallVector<StringRef, 5> dirs;
     CIncludeDirs.split(dirs, ":");
     for (StringRef dir : dirs) {
--- clang/lib/Driver/ToolChains/Gnu.cpp	2019-10-26 09:49:27.003752743 +0200
+++ clang/lib/Driver/ToolChains/Gnu.cpp	2019-10-26 09:50:49.067236497 +0200
@@ -1743,7 +1743,7 @@
   // If we have a SysRoot, ignore GCC_INSTALL_PREFIX.
   // GCC_INSTALL_PREFIX specifies the gcc installation for the default
   // sysroot and is likely not valid with a different sysroot.
-  if (!SysRoot.empty())
+  if (!(SysRoot.empty() || SysRoot == "/"))
     return "";
 
   return GCC_INSTALL_PREFIX;