Converted TestConstVariables expected failure to @expectedFailureClang.
Converted TestConstVariables expected failure to @expectedFailureClang.

TestConstVariables.py was disabled on Linux and marked as
unconditional expected failure. This change removes the Linux disable
and makes the expected failure conditional on using clang. The test
runs fine on gcc-built lldb.



git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@202514 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb