cmd/vet: include function name or value in copylock message

cmd/vet: include function name or value in copylock message

Given
	var t struct{ lock sync.Mutex }
	var fntab []func(t)
	f(a(), b(&t), c(), fntab[0](t))

Before:
	function call copies lock value: struct{lock sync.Mutex} contains sync.Mutex

After:
	call of fntab[0] copies lock value: struct{lock sync.Mutex} contains sync.Mutex

This will make diagnosis easier when there are multiple function calls per line.

Change-Id: I9881713c5671b847b84a0df0115f57e7cba17d72
Reviewed-on: https://go-review.googlesource.com/34730
Reviewed-by: Ian Lance Taylor <iant@golang.org>