[MLIR][CAPI] Move `DenseMapInfo<MlirTypeID>`

I mistakenly put this in `mlir/CAPI/Support.h` at some point during the flurry of refactoring of `TypeCaster`s but as @jpienaar rightly pointed out, it doesn't belong there.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D151669