[MC] Make ELFEntrySizeMap a DenseMap (#91728)

There is no need for an ordered std::map and also no need to duplicate
the section name, which is owned by the ELFSectionKey. Therefore, use a
DenseMap instead and don't copy the string. As a further, minor
performance optimization, avoid the hash table lookup in
isELFGenericMergeableSection when the section name was just added.

This slightly improves compilation performance in our application, where
we occasionally compile many small object files.