[llvm] Add a SFINAE template parameter to DenseMapInfo

This allows for using SFINAE partial specialization for DenseMapInfo.
In MLIR, this is particularly useful as it will allow for defining partial
specializations that support all Attribute, Op, and Type classes without
needing to specialize DenseMapInfo for each individual class.

Differential Revision: https://reviews.llvm.org/D113641

GitOrigin-RevId: 4c484f11d355e4293f7b245a9330f0a1e89630ac
1 file changed