Fix deduction of __atomic_load's parameter types.

Summary:
__atomic_load's allows it's first argument to be a pointer to a const type. However the second argument is an output parameter and must be a pointer to non-const.
This patch fixes the signature of __atomic_load generated by clang so that it respects the above requirements.

Reviewers: rsmith, majnemer

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13420

llvm-svn: 264967