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

Test/hlsl.coverage.frag

@@ -0,0 +1,20 @@
+
+// Verify that coverage mask is an array, as required by SPIR-V.
+
+struct PS_INPUT
+{
+};
+
+struct PS_OUTPUT
+{
+    float4 vColor : SV_Target0;
+    uint nCoverageMask : SV_Coverage;
+};
+
+PS_OUTPUT main( PS_INPUT i )
+{
+    PS_OUTPUT o;
+    o.vColor = float4(1.0, 0.0, 0.0, 1.0);
+    o.nCoverageMask = 0;
+    return o;
+}