[OpenMP] Add NoSync attributes to alloc / free shared RTL calls

This patch adds the `nosync` attribute to the `__kmpc_alloc_shared` and
`__kmpc_free_shared` runtime library calls. This allows code analysis to
know that these functins dont contain any barriers. This will help
optimizations reason about the CFG of blocks containing these calls.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D109995