Fixes#18840: too large closure of mesa_drivers.
Tested atop 16.09:
- clang compiles a hello-world app;
- mesa seems to link OK;
- ispc builds.
Size comparison:
- 80 MB of full llvm-3.7 on 16.03;
- 200 MB of full llvm-3.9 on 16.09 before this patch;
- 50 MB of libLLVM after this commit.
