This patch implements 'shared' variables in compute shader on D3D11
back-ends. GLSL shared variables are translated into 'groupshared'
ones in HLSL.

Note that although HLSL allows initializing the variables with
'groupshared' qualifier, currently we do not initialize them because:
1. It is very slow to for d3d11 drivers to compile the compute shader if
   we add the code to initialize a shared variable with large array size.
2. It seems unnecessary to do so and in GLSL it is not allowed to
   initialize a shared variable in the declaration. (ESSL 3.1, Chapter
   4.3.8)

BUG=angleproject:2682
TEST=angle_end2end_tests

Change-Id: Ica8247e1b98059968612a36e369718ef113a598c
Reviewed-on: https://chromium-review.googlesource.com/1109587Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>