warnings in source/Plugins/Language/ObjC/NSArray.cpp

r310959 contains GNU extensions in source/Plugins/Language/ObjC/NSArray.cpp.
Building with clang 3.8 and -WError, I see:

/local/mnt/workspace/bots/llvmhexbot-sles11_sd_48/hexagon-clang-83/build/llv
m/tools/lldb/source/Plugins/Language/ObjC/NSArray.cpp:181:7: error:
anonymous structs are a GNU extension [-Werror,-Wgnu-anonymous-struct]
      struct {
      ^
/local/mnt/workspace/bots/llvmhexbot-sles11_sd_48/hexagon-clang-83/build/llv
m/tools/lldb/source/Plugins/Language/ObjC/NSArray.cpp:181:7: error:
anonymous types declared in an anonymous union are an extension
[-Werror,-Wnested-anon-types]

The patch is to handle new MacOS 10.13 internal layouts. Could someone at
Apple please clean this up?