[OpenCL] Added underscores to the names of 'to_addr' OpenCL built-ins.

Summary:
In order to re-define OpenCL built-in functions
'to_{private,local,global}' in OpenCL run-time library LLVM names must
be different from the clang built-in function names.

Reviewers: yaxunl, Anastasia

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23120

llvm-svn: 277743