[llvm] [MC] Make ELFEntrySizeMap a DenseMap (PR #91728)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

ELFEntrySizeKey is from https://reviews.llvm.org/D72194 but it is not iterated, so no ordering requirement.

https://github.com/llvm/llvm-project/pull/91728