[OpenMP] Stable sort Privates to remove non-deterministic ordering

Summary:
This fixes the following failures uncovered by D39245:
    Clang :: OpenMP/task_firstprivate_codegen.cpp
    Clang :: OpenMP/task_private_codegen.cpp
    Clang :: OpenMP/taskloop_firstprivate_codegen.cpp
    Clang :: OpenMP/taskloop_lastprivate_codegen.cpp
    Clang :: OpenMP/taskloop_private_codegen.cpp
    Clang :: OpenMP/taskloop_simd_firstprivate_codegen.cpp
    Clang :: OpenMP/taskloop_simd_lastprivate_codegen.cpp
    Clang :: OpenMP/taskloop_simd_private_codegen.cpp

Reviewers: rjmccall, ABataev, AndreyChurbanov

Reviewed By: rjmccall, ABataev

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D39947

llvm-svn: 319222