go.tools/go/ssa: record ast.CallExpr.Rparen as Alloc.Pos() for varargs arrays.

+ pointer analysis test-case.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/108980043

go/pointer/testdata/func.go

@@ -152,6 +152,10 @@ func func7() {
 	// @calls main.func7 -> (main.D).f$thunk
 }
 
+func func8(x ...int) {
+	print(&x[0]) // @pointsto varargs[*]@varargs:15
+}
+
 func main() {
 	func1()
 	func2()
@@ -160,6 +164,7 @@ func main() {
 	func5()
 	func6()
 	func7()
+	func8(1, 2, 3) // @line varargs
 }
 
 // @calls <root> -> main.main

go/ssa/builder.go

@@ -854,8 +854,8 @@ func (b *builder) emitCallArgs(fn *Function, sig *types.Signature, e *ast.CallEx
 		} else {
 			// Replace a suffix of args with a slice containing it.
 			at := types.NewArray(vt, int64(len(varargs)))
-			// Don't set pos for implicit Allocs.
 			a := emitNew(fn, at, token.NoPos)
+			a.setPos(e.Rparen)
 			a.Comment = "varargs"
 			for i, arg := range varargs {
 				iaddr := &IndexAddr{

go/ssa/ssa.go

@@ -474,7 +474,7 @@ type Builtin struct {
 // instantiate these types.
 //
 // Pos() returns the ast.CompositeLit.Lbrace for a composite literal,
-// or the ast.CallExpr.Lparen for a call to new() or for a call that
+// or the ast.CallExpr.Rparen for a call to new() or for a call that
 // allocates a varargs slice.
 //
 // Example printed form: