HLSL: allow "sample" as a valid identifier.

HLSL has keywords for various interpolation modifiers such as "linear",
"centroid", "sample", etc.  Of these, "sample" appears to be special,
as it is also accepted as an identifier string, where the others are not.

This PR adds this ability, so the construct "int sample = 42;" no longer
produces a compilation error.

New test = hlsl.identifier.sample.frag

Test/hlsl.identifier.sample.frag

@@ -0,0 +1,18 @@
+
+struct MyStruct {
+    sample        float a;
+    noperspective float b;
+    linear        float c;
+    centroid      float d;
+};
+
+int sample(int x) { return x; } // HLSL allows this as an identifier as well.
+
+float4 main() : SV_Target0
+{
+    // HLSL allows this as an identifier as well.
+    // However, this is not true of other qualifier keywords such as "linear".
+    int sample = 3;
+
+    return float4(0,0,0,0);
+}