Rename LLVM's single use of CALLBACK as a template parameter to work around the Windows SDK #define CALLBACK __stdcall
I'm sympathetic to the Michael Bolton argument but these are the only 2 lines affected in all of LLVM.
Once accepted I'll need someone to commit the change on my behalf.
Good catch on fixing this! I think a better fix would be to change the name CALLBACK to Callback to more closely match our usual naming conventions (https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly). Keeping it in all caps makes the identifier look like a macro when it isn't one. You might as well address the clang-format issue while you're touching the code, as well.