HLSL: Flesh out misc. declaration grammar: semantics/registers/annotations/precise/etc.

Details within these bear even more fleshing out, but would like to have
that driven by actual need.

Test/hlsl.struct.frag

@@ -16,6 +16,17 @@ struct {
     float4 i;
 } s2;
 
+struct {
+    linear float4 a;
+    nointerpolation bool b;
+    noperspective centroid float1 c;
+    sample centroid float2 d;
+    bool ff1 : SV_IsFrontFace;
+    bool ff2 : packoffset(c0.y);
+    bool ff3 : packoffset(c0.y) : register(ps_5_0, s[0]) ;
+    float4 ff4 : VPOS : packoffset(c0.y) : register(ps_5_0, s[0]) <int bambam=30;> ;
+} s4;
+
 float4 PixelShaderFunction(float4 input) : COLOR0
 {
     struct FS {
@@ -23,4 +34,6 @@ float4 PixelShaderFunction(float4 input) : COLOR0
     } s3;
 
     s3 == s3;
+
+    return input;
 }