[HLSL] Implement RWBuffer::operator[] via __builtin_hlsl_resource_getpointer (#117017)

This introduces `__builtin_hlsl_resource_getpointer`, which lowers to
`llvm.dx.resource.getpointer` and is used to implement indexing into
resources.

This will only work through the backend for typed buffers at this point,
but the changes to structured buffers should be correct as far as the
frontend is concerned.

Note: We probably want this to return a reference in the HLSL device
address space, but for now we're just using address space 0. Creating a
device address space and updating this code can be done later as
necessary.

Fixes #95956

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