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...