[llvm] Split out DenseMapInfo<variant> specialization

Remove the `DenseMapInfo<std::variant<Ts...>>` variant out from
`llvm/ADT/DenseMapInfo.h` into a separate header
`llvm/ADT/DenseMapInfoVariant.h`

This allows us to remove the `<variant>` include, which is being
transitively and unncessary included in all translation units that
include `llvm/ADT/DenseMap.h`.

There have been similar changes to move out specializations for

    * `APInt.h` fd7e309e and
    * `StringRef.h`/`ArrayRef.h`
      983565a6

to reduce the compilation time. As we are unable to move the
specialization into `<variant>`, we create a separate
`DenseMapInfoVariant.h` header that can be used by anyone who needs this
specialization.

This reduces the total number of preprocessing tokens across the LLVM
source files in lib from (roughly) 1,964,876,961 to 1,936,551,496 - a
reduction of ~1.44%. This should result in a small improvement in
compilation time.

Differential Revision: https://reviews.llvm.org/D150997