Support the tls_model attribute (PR9788)

Support the tls_model attribute (PR9788)

This adds support for the tls_model attribute. This allows the user to
choose a TLS model that is better than what LLVM would select by
default. For example, a variable might be declared as:

  __thread int x __attribute__((tls_model("initial-exec")));

if it will not be used in a shared library that is dlopen'ed.

This depends on LLVM r159077.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159078 91177308-0d34-0410-b5e6-96231b3b80d8