[AMDGPU] Emit debugger prologue and emit the rest of the debugger fields in the kernel code header

Debugger prologue is emitted if -mattr=+amdgpu-debugger-emit-prologue.

Debugger prologue writes work group IDs and work item IDs to scratch memory at fixed location in the following format:
  - offset 0: work group ID x
  - offset 4: work group ID y
  - offset 8: work group ID z
  - offset 16: work item ID x
  - offset 20: work item ID y
  - offset 24: work item ID z

Set
  - amd_kernel_code_t::debug_wavefront_private_segment_offset_sgpr to scratch wave offset reg
  - amd_kernel_code_t::debug_private_segment_buffer_sgpr to scratch rsrc reg
  - amd_kernel_code_t::is_debug_supported to true if all debugger features are enabled

Differential Revision: http://reviews.llvm.org/D20335

llvm-svn: 273769