mesa: use llvm-3.9 instead of 3.8

(cherry picked from commit 040b941)
No problems reported so far, and we've got a couple weeks to stabilize
anyway. It seems required to support some new GPUs, #17991.

pkgs/top-level/all-packages.nix

@@ -8768,7 +8768,7 @@ in
     # makes it slower, but during runtime we link against just mesa_drivers
     # through /run/opengl-driver*, which is overriden according to config.grsecurity
     grsecEnabled = true;
-    llvmPackages = llvmPackages_38; # various problems with 3.7; see #11367, #11467
+    llvmPackages = llvmPackages_39;
   });
   mesa_glu =  mesaDarwinOr (callPackage ../development/libraries/mesa-glu { });
   mesa_drivers = mesaDarwinOr (