[mlir][DenseElementsAttr] Allow for custom floating point types in `getValues`

Some users have native c++ data types that correspond to floating point values stored within a DenseElementsAttr that do not have a corresponding native C++ data type(e.g. bfloat16/half/etc). This revision allows for such users to use those native types directly, and removes the need to go through APFloat when the much faster native value path is available.

Differential Revision: https://reviews.llvm.org/D91402