Currently, `__constant__` variables do not get unconditionally marked as
`constant` in IR, which seems a bit odd given their definition. This is
generally inconsequential for NVPTX/AMDGPU, since said variables get
emitted in the constant address space for those BEs. However, it is
potentially significant for e.g. HIP-on-SPIR-V cases, as SPIR-V does not
allow casts to/from the constant AS (`UniformConstant`), which forces
`__constant__` variables to be emitted in the global AS, thus making IR
constness meaningful.
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