HLSL: require coverage mask to be arrayed output.

SPIR-V requires the coverage mask to be an array of integers, but HLSL
allows scalar integers.  This adds the requisite type conversion and
wrapped entry point handling.

Fixes: #1202

gtests/Hlsl.FromFile.cpp

@@ -148,6 +148,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.constructArray.vert", "main"},
         {"hlsl.constructexpr.frag", "main"},
         {"hlsl.constructimat.frag", "main"},
+        {"hlsl.coverage.frag", "main"},
         {"hlsl.depthGreater.frag", "PixelShaderFunction"},
         {"hlsl.depthLess.frag", "PixelShaderFunction"},
         {"hlsl.discard.frag", "PixelShaderFunction"},