[llvm] DenseMap: support enum class keys (PR #95972)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Though I would like to hear where you plan to use this. Usually enums imply that there is a reasonably small, continuously numbered and fixed set of keys, in which case using a vector instead of map is often preferable.

This is a requirement for my new Presburger parser, which keeps a map of `{DimOrSymbol, position} -> multiplier`. The parser isn't ready just yet, but will be soon'ish.

https://github.com/llvm/llvm-project/pull/95972