Don't build switch lookup tables for dllimport or TLS variables

We would previously put dllimport variables in switch lookup tables, which
doesn't work because the address cannot be used in a constant initializer.
This is basically the same problem that we have in PR19955.

Putting TLS variables in switch tables also desn't work, because the
address of such a variable is not constant.

Differential Revision: http://reviews.llvm.org/D4220