[AMDGPU][NFC] Add DWARF extension support for SIMD execution

- Add current iteration to the context of a DWARF expression evaluation.
- Add DW_AT_LLVM_iterations attribute to specify the number of
  iterations executing concurrently.
- Add DF_OP_LLVM_push_iteration to support optimizations that result in
  multiple iterations executing concurrently.
- Add DW_OP_LLVM_overlay and DW_OP_LLVM_bit_overlay to support
  expressing the location of arrays that are promoted to vector
  registers in SIMD vectorized loops.
- Generally clarify the difference between SIMT and SIMD execution.
- Change the DW_AT_LLVM_active_lane attribute to take location
  description expression so that a loclist can be used to express
  different vales at different program locations.

Reviewed By: scott.linder

Differential Revision: https://reviews.llvm.org/D117572