Converted TestRegisterVariables to run on Linux and skip with clang.

TestRegisterVariables.test_with_dwarf_and_run_command was being
skipped on Linux due to issues with clang. I converted it to a
@expectedFailureClang and no longer skip on Linux.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202515 91177308-0d34-0410-b5e6-96231b3b80d8