This patch adds the support of translating atomicExchange and
atomicCompSwap without return value on D3D11 back-ends.

As the last parameter of the HLSL intrinsic functions
InterlockedExchange and InterlockedCompareExchange is not optional,
when there is a call of either atomicExchange or atomicCompSwap
without return value, we add a temporary variable for it, so that
we can directly translate all of such calls in outputHLSL.cpp.

BUG=angleproject:2682
TEST=angle_end2end_tests

Change-Id: I7e9c6d3c7d1846c865909b2f5a26592846c82582
Reviewed-on: https://chromium-review.googlesource.com/1161744Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>