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

gtests/Hlsl.FromFile.cpp

@@ -118,6 +118,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.getdimensions.rw.dx10.frag", "main"},
         {"hlsl.getdimensions.dx10.vert", "main"},
         {"hlsl.getsampleposition.dx10.frag", "main"},
+        {"hlsl.identifier.sample.frag", "main"},
         {"hlsl.if.frag", "PixelShaderFunction"},
         {"hlsl.inoutquals.frag", "main"},
         {"hlsl.init.frag", "ShaderFunction"},