[NFC] Refactor dxil metadata code

DXIL relies on a whole bunch of IR metadata constructs being populated
in the right shape. Rather than just hard coding or using complicated
arrangements of constant data strings, let's make first-class objects
that reprensent the metadata and manage reading and writing the
metadata from the module.

Reviewed By: python3kgae

Differential Revision: https://reviews.llvm.org/D134397