[AMDGPU] Fix prologue/epilogue markers in .debug_line table for trivial functions

All the prologue instructions should have unknown source location
co-ordinates while the epilogue instructions should have source
location of last non-debug instruction after which epilogue
instructions are insrted.

This ensures the prologue/epilogue markers are generated correctly
in the line table.

Changes are brought in from the downstream CFI patches.

Reviewed By: scott.linder

Differential Revision: https://reviews.llvm.org/D131485