go/callgraph/vta/internal/trie: fix build with go1.12

- Convert shift count to unsigned to make go1.12 happy.

https://github.com/golang/proposal/blob/master/design/19113-signed-shift-counts.md

- Require go1.13+ for tests that use the new language features introduced
in go1.13.

https://golang.org/doc/go1.13#language

Change-Id: I30dc5ee44ccf1a76ce30f352751316efdebc5929
Reviewed-on: https://go-review.googlesource.com/c/tools/+/365634
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tim King <taking@google.com>

go/callgraph/vta/internal/trie/bits.go

@@ -46,7 +46,7 @@ func branchingBit(x, y prefix) bitpos {
 	if p == 0 {
 		return 0
 	}
-	return 1 << (bits.Len64(uint64(p)) - 1)
+	return bitpos(1) << uint(bits.Len64(uint64(p))-1) // uint conversion needed for go1.12
 }
 
 // zeroBit returns true if k has a 0 bit at position `b`.

go/callgraph/vta/internal/trie/bits_test.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.13
+// +build go1.13
+
 package trie
 
 import (

go/callgraph/vta/internal/trie/trie_test.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.13
+// +build go1.13
+
 package trie
 
 import (