[lldb][NFC] Fix build with GCC<6

GCC/libstdc++ before 6.1 can't handle scoped enums as unordered_map keys. LLVM
(and some build) bots officially support some GCC 5.x versions, so this patch
just makes the enum unscoped until we can require GCC 6.x.

GitOrigin-RevId: 37510f69b4cb8d76064f108d57bebe95984a23ae
1 file changed