Fixes#141840
This PR implements support for the `memcpy` intrinsic in the DXIL
CBuffer Access pass with the following restrictions:
- The CBuffer Access must be the `src` operand of `memcpy` and must be
direct (i.e., not a GEP)
- The type of the CBuffer Access must be of an Array Type
These restrictions greatly simplify the implementation of `memcpy` yet
still covers the known uses in DML shaders.
Furthermore, to prevent errors like #141840 from occurring silently
again, this PR adds error reporting for unsupported users of globals in
the DXIL CBuffer Access pass.
This introduces a pass that walks accesses to globals in cbuffers and
replaces them with accesses via the cbuffer handle itself. The logic to
interpret the cbuffer metadata is kept in `lib/Frontend/HLSL` so that it
can be reused by other consumers of that metadata.
Fixes#124630.
