[SCF] Add thread_dim_mapping attribute to scf.foreach_thread

An optional thread_dim_mapping index array attribute specifies for each
virtual thread dimension, how it remaps 1-1 to a set of concrete processing
element resources (e.g. a CUDA grid dimension or a level of concrete nested
async parallelism). At this time, the specification is backend-dependent and
is not verified by the op, beyond being an index array attribute.
It is the reponsibility of the lowering to interpret the index array in the
context of the concrete target the op is lowered to, or to ignore it when
the specification is ill-formed or unsupported for a particular target.

Differential Revision: https://reviews.llvm.org/D128633