go: add «1863. Sum of All Subset XOR Totals»

URL:	https://leetcode.com/problems/sum-of-all-subset-xor-totals/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/sum-of-all-subset-xor-totals.go

@@ -0,0 +1,9 @@
+package main
+
+func subsetXORSum(nums []int) int {
+	result := 0
+	for _, x := range nums {
+		result |= x
+	}
+	return result << (len(nums) - 1)
+}