HLSL: Add namespace grammar and some basic semantics.

Unknown how extensive the semantics need to be yet. Need real
feedback from workloads. This is just done as part of unifying it
with the class/struct namespaces and grammar productions.

gtests/Hlsl.FromFile.cpp

@@ -161,6 +161,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.logical.unary.frag", "main"},
         {"hlsl.logical.binary.frag", "main"},
         {"hlsl.logical.binary.vec.frag", "main"},
+        {"hlsl.namespace.frag", "main"},
         {"hlsl.matNx1.frag", "main"},
         {"hlsl.matrixSwizzle.vert", "ShaderFunction"},
         {"hlsl.mintypes.frag", "main"},