[AMDGPU] Set the default globals address space to 1

This will ensure that passes that add new global variables will create them
in address space 1 once the passes have been updated to no longer default
to the implicit address space zero.
This also changes AutoUpgrade.cpp to add -G1 to the DataLayout if it wasn't
already to present to ensure bitcode backwards compatibility.

Reviewed by: arsenm

Differential Revision: https://reviews.llvm.org/D84345