[OpenMP] Fix declare target link implementation

Summary:
This patch fixes the case where variables in different compilation units or the same compilation unit are under the declare target link clause AND have the same name.
This also fixes the name clash error that occurs when unified memory is activated.
The changes in this patch include:
- Pointers to internal variables are given unique names.
- Externally visible variables are given the same name as before.
- All pointer variables (external or internal) are weakly linked.

Reviewers: ABataev, jdoerfert, caomhin

Reviewed By: ABataev

Subscribers: lebedev.ri, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64592

llvm-svn: 367613