This captures the target shader model and pipeline stage into the DXIL
metadata for consumption by the DirectX runtime.
Reviewed By: python3kgae
Differential Revision: https://reviews.llvm.org/D134469
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