HLSL: Partially address issue #463: accept GLSL layout(...).

This includes all "per variable" layout qualifiers, but the
key ones mattering and tested for now are:
  set=
  binding=
  constant_id=
  push_constant

Test/hlsl.layout.frag

@@ -0,0 +1,14 @@
+layout(set=3,binding=5) tbuffer tbufName {
+    layout(offset = 16) float4 v1;
+};
+
+layout(push_constant) tbuffer tbufName2 {
+    float4 v5;
+};
+
+layout(constant_id=17) const int specConst = 10;
+
+float4 PixelShaderFunction(float4 input) : COLOR0
+{
+    return input + v1 + v5;
+}