[Internalize] Preserve variables externally initialized.

- ``externally_initialized`` variables would be initialized or modified
  elsewhere. Particularly, CUDA or HIP may have host code to initialize
  or modify ``externally_initialized`` device variables, which may not
  be explicitly referenced on the device side but may still be used
  through the host side interfaces. Not preserving them triggers the
  elimination of them in the GlobalDCE and breaks the user code.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D105135