Silence -Wreturn-type with gcc 5.2
Silence -Wreturn-type with gcc 5.2

The switch is fully covered, mark "default" as unreachable.  NFC.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@250667 91177308-0d34-0410-b5e6-96231b3b80d8

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