[DirectX] Follow naming conventions for enumerators in DXILABI.h. NFC (#86237)

These all-caps names differ from the llvm naming conventions for no good
reason, and `VOID` in all caps can cause problems in windows
environments (see [1]). Rename them to UpperCamelCase.

[1]: https://github.com/clangd/clangd/issues/1983