Bug#848368: llvm-toolchain-3.9: Please add ELF symbols versions to the libraries

[Rebecca N. Palmer]

Looks like this is a known cowdancer issue (#640684 - fixed only for 
cowbuilder --build, not --login); continuing with LD_PRELOAD= 
debian/rules build, the LLVM tests pass:

   Expected Passes    : 16742
   Expected Failures  : 124
   Unsupported Tests  : 339
   Unexpected Passes  : 17

Still building the clang tests...