Support wider range of expressions in enum discriminants

tests/rust/enum_discriminant.rs

@@ -0,0 +1,15 @@
+pub const FOUR: i8 = 4;
+
+#[repr(i8)]
+enum E {
+    A = 1,
+    B = -1,
+    C = 1 + 2,
+    D = FOUR,
+    F = (5),
+    G = '6' as i8,
+    H = false as i8,
+}
+
+#[no_mangle]
+pub extern "C" fn root(_: &E) {}