dmUpdate: move type declaration and change type for nvhpc

[rem1776]

Description
fixes compilation errors related to the diag manager updates when building with nvhpc. Had to move a type declaration to be before any references to it and switch a class to a type.

I still ran into runtime errors when running the unit testing with nvhpc.

How Has This Been Tested?
built with nvhpc on amd box (23.11) and gaea (22.3)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes