[CUDA][HIP] Skip setting `externally_initialized` for static device variables.

Summary:
- By declaring device variables as `static`, we assume they won't be
  addressable from the host side. Thus, no `externally_initialized` is
  required.

Reviewers: yaxunl

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62603

llvm-svn: 361994