Fixed up TestConstVariables for Linux/FreeBSD, take 2.

Marked XFAIL for:
clang - all versions < 3.5.  I verified the first version it passes on Linux is 3.5.
clang - version 3.7 (verified this fails)
clang - TOT/3.8 (verified this currently fails)

Ideally we dig into why this started failing and (hopefully) get this passing again.

llvm-svn: 247664