[HIP] Fix visibility of `__constant__` variables.

Summary:
- `__constant__` variables should not be `hidden` as the linker may turn
  them into `LOCAL` symbols.

Reviewers: yaxunl

Subscribers: jvesely, nhaehnle, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61194

llvm-svn: 359344