cmd/compile: fix lexical scope of escaped variables

When a local variable is moved to the heap the declaration position
should be preserved so that later on we can assign it to the correct
DW_TAG_lexical_block.

Fixes #20959

Change-Id: I3700ef53c68ccd506d0633f11374ad88a52b2898
Reviewed-on: https://go-review.googlesource.com/47852
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -2100,6 +2100,7 @@ func moveToHeap(n *Node) {
 	heapaddr := temp(types.NewPtr(n.Type))
 	heapaddr.Sym = lookup("&" + n.Sym.Name)
 	heapaddr.Orig.Sym = heapaddr.Sym
+	heapaddr.Pos = n.Pos
 
 	// Unset AutoTemp to persist the &foo variable name through SSA to
 	// liveness analysis.

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

@@ -49,6 +49,7 @@ var testfile = []testline{
 	{line: "func f4(x int) { }"},
 	{line: "func f5(x int) { }"},
 	{line: "func f6(x int) { }"},
+	{line: "func fi(x interface{}) { if a, ok := x.(error); ok { a.Error() } }"},
 	{line: "func gret1() int { return 2 }"},
 	{line: "func gretbool() bool { return true }"},
 	{line: "func gret3() (int, int, int) { return 0, 1, 2 }"},
@@ -163,6 +164,15 @@ var testfile = []testline{
 	{line: "	}"},
 	{line: "	f(3); f1(b)"},
 	{line: "}"},
+	{line: "func TestEscape() {"},
+	{line: "	a := 1", vars: []string{"var a int"}},
+	{line: "	{"},
+	{line: "		b := 2", scopes: []int{1}, vars: []string{"var &b *int", "var p *int"}},
+	{line: "		p := &b", scopes: []int{1}},
+	{line: "		f1(a)", scopes: []int{1}},
+	{line: "		fi(p)", scopes: []int{1}},
+	{line: "	}"},
+	{line: "}"},
 	{line: "func main() {"},
 	{line: "	TestNestedFor()"},
 	{line: "	TestOas2()"},
@@ -173,6 +183,7 @@ var testfile = []testline{
 	{line: "	TestBlock()"},
 	{line: "	TestDiscontiguousRanges()"},
 	{line: "	TestClosureScope()"},
+	{line: "	TestEscape()"},
 	{line: "}"},
 }