mesa drivers: revert back to llvm-3.6 for now

/cc #11367, #11467.
Only the drivers are overridden for now to avoid a mass rebuild.

pkgs/top-level/all-packages.nix

@@ -7805,6 +7805,7 @@ let
   mesa_drivers = mesaDarwinOr (
     let mo = mesa_noglu.override {
       grsecEnabled = config.grsecurity or false;
+      llvmPackages = llvmPackages_36; # various problems with 3.7; see #11367, #11467
     };
     in mo.drivers
   );