[Assignment Tracking][9/*] Don't drop DIAssignID in dropUnknownNonDebugMetadata

The Assignment Tracking debug-info feature is outlined in this RFC:

https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir

A DIAssignID attachment is debug metadata, so don't drop it.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133292