[clang] [Clang][Diagnostics] Update select uses in DiagnosticXKinds.td to use enum_select (PR #130868)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

The parts I could see are in teh right direction, I've made some comments.

Please undo the clang-format of `ExprConstant.cpp` (and only clang-format the parts of the file you modified) and I can look at the hwole thing.

https://github.com/llvm/llvm-project/pull/130868