[AMDGPU] Update DWARF proposal

- Unify the sections on DWARF expression and location lists.

- Allow a location description to have one or more single location
  descriptions.

- Define context of DWARF expression that includes an initial
  stack. Allow initial stack to be used when evaluating location list
  expression with overlapping PC ranges.

- Reorganize the DWARF proposal in AMDGPUUsage so suitable for
  submission to the DWARF site.

- Replace CFI instruction DW_CFA_LLVM_def_cfa_aspace with
  DW_CFA_def_aspace_cfa and DW_CFA_def_aspace_cfa_sf. This is to avoid
  the problem that DW_CFA_def_cfa and DW_CFA_def_cfa_sf cannot use a
  register that is not the size of an address in the CFA address
  space.

- Clarify DWARF address class and DWARF address space. Define language
  values for DWARF address classes and specify how they are used by
  some common source languages.

- Define rules for accessing registers and derefencing memory when the
  type size and register size or byte size operand do not match.

- Numerous cleanups for consistency.

Differential Revision: https://reviews.llvm.org/D70523