Remove decorator for now-passing test

The underlying issue was actually a Clang bug, now fixed.  The test now
reports XPASS for me locally and on the buildbot.

llvm.org/pr17183 (LLDB)
llvm.org/pr18950 (Clang)


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@206761 91177308-0d34-0410-b5e6-96231b3b80d8