merge two near-identical functions createPrivateGlobalForString into one

Summary:
We have two copies of createPrivateGlobalForString (in asan and in esan).
This change merges them into one. NFC

Reviewers: vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53178

llvm-svn: 344314