HLSL: Implement proper nesting of symbol-table scopes and identifier searching.

Test/hlsl.scope.frag

@@ -0,0 +1,30 @@
+float4 PixelShaderFunction(float4 input) : COLOR0
+{
+    int x;
+    x;
+    {
+        float x;
+        x;
+        {
+            bool x;
+            x;
+            {
+                float3 x;
+                x;
+            }
+            x;
+        }
+        x;
+    }
+    x;
+
+    if (x > 0)
+        bool x;
+
+    while (x > 0)
+        bool x;
+
+    do {
+        bool x;
+    } while (x > 0);
+}