SPV: Fix #807: use --hlsl-offsets to allow hlsl-style offsets in a buffer.

Corresponds to the EShMsgHlslOffsets flag in messages.
Works for both GLSL and HLSL.

Test/hlsl.hlslOffset.vert

@@ -0,0 +1,20 @@
+cbuffer b {
+    float m0;
+    float3 m4;
+    //////
+    float m16;
+    float3 m20 : packoffset(c1.y);
+    /////
+    float3 m36 : packoffset(c2.y);
+    /////
+    float2 m56 : packoffset(c3.z);
+    /////
+    float m64;
+    float2 m68;
+    float m76;
+    //////
+    float m80;
+    float2 m96[1];
+};
+
+void main() {}