[AMDGPU] DWARF proposal changes for expression context

- Clarify what context is used in DWARF expression evaluation.

- Define location descriptions to fully resolve the context and so
  include the context in their result.

- As a consequence of location descriptions being fully resoved,
  change address spaces so only a swizzled and unswizzled private
  address space is defined. The lane is now part of the location
  description context.

- Clarify how call frame information is used to fully resolve
  expressions that specify registers.

Reviewed By: scott.linder

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