cmd/compile: attach correct line number for fallthru-return

Fixes #14646.

Change-Id: I0bb82ed6d3533633cd8369ba37aa467948bbe155
Reviewed-on: https://go-review.googlesource.com/20381
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -180,6 +180,7 @@ func buildssa(fn *Node) *ssa.Func {
 		s.stmts(s.exitCode)
 		m := s.mem()
 		b := s.endBlock()
+		b.Line = fn.Func.Endlineno
 		b.Kind = ssa.BlockRet
 		b.Control = m
 	}

test/fixedbugs/issue14646.go

+// run
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "runtime"
+
+func main() {
+	var file string
+	var line int
+	func() {
+		defer func() {
+			_, file, line, _ = runtime.Caller(1)
+		}()
+	}() // this is the expected line
+	const EXPECTED = 18
+	if line != EXPECTED {
+		println("Expected line =", EXPECTED, "but got line =", line, "and file =", file)
+	}
+}