Add support for enums.

tests/enum-8.ispc

@@ -0,0 +1,26 @@
+
+enum Foo {
+    ZERO,
+    ONE,
+    TWO,
+    TEN = 10,
+    ELEVEN
+};
+
+enum Bar {
+    FIFTY = 50,
+    THOUSAND = 1000
+};
+
+export uniform int width() { return programCount; }
+
+Foo Func(Foo f) { return ++f; }
+Bar Func(Bar b) { return FIFTY; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    RET[programIndex] = (int)Func(ONE);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2;
+}