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/spv.hlslOffsets.vert

@@ -0,0 +1,27 @@
+#version 450
+
+buffer block {
+    float m0;
+    vec3 m4;
+    //////
+    float m16;
+    layout(offset=20) vec3 m20;
+    /////
+    vec3 m32;
+    /////
+    vec2 m48;
+    vec2 m56;
+    ////
+    float m64;
+    vec2 m68;
+    float m76;
+    //////
+    float m80;
+    layout(offset=88) vec2 m88;
+    //////
+    vec2 m96;
+    ///////
+    dvec2 m112;
+};
+
+void main() {}