[dev.regabi] all: merge master (bf0f7c9) into dev.regabi

This merge involved two merge conflicts:

1. walk's ascompatee code has been substantially refactored on
dev.regabi, so CL 285633 is ported to the new style.

2. The os.TestDirFS workaround added in CL 286213 can be removed now
that #42637 has been fixed by CL 285720.

Conflicts:

- src/cmd/compile/internal/gc/walk.go
- src/os/os_test.go

Merge List:

+ 2021-01-25 bf0f7c9d78 doc/go1.16: mention os.DirFS in os section
+ 2021-01-25 deaf29a8a8 cmd/compile: fix order-of-assignment issue w/ defers
+ 2021-01-25 ad2ca26a52 doc/go1.16: mention os.DirEntry and types moved from os to io/fs
+ 2021-01-25 a51921fa5b doc/go1.16: mention new testing/iotest functions
+ 2021-01-25 e6b6d107f7 doc/go1.16: mention deprecation of io/ioutil
+ 2021-01-25 96a276363b doc/go1.16: mention go/build changes
+ 2021-01-25 3d85c69a0b html/template: revert "avoid race when escaping updates template"
+ 2021-01-25 54514c6b28 cmd/go: fix TestScript/cgo_path, cgo_path_space when CC set
+ 2021-01-25 6de8443f3b doc/asm: add a section on go_asm.h, clean up go_tls.h section
+ 2021-01-25 54b251f542 lib/time, time/tzdata: update tzdata to 2021a
+ 2021-01-25 ff82cc971a os: force consistent mtime before running fstest on directory on Windows
+ 2021-01-25 044f937a73 doc/go1.16: fix WalkDir and Walk links
+ 2021-01-23 b634f5d97a doc/go1.16: add crypto/x509 memory optimization
+ 2021-01-23 9897655c61 doc/go1.16: reword ambiguously parsable sentence
+ 2021-01-23 cd99385ff4 cmd/internal/obj/arm64: fix VMOVQ instruction encoding error
+ 2021-01-23 66ee8b158f runtime: restore cgo_import_dynamic for libc.so on openbsd
+ 2021-01-22 25c39e4fb5 io/ioutil: fix example test for WriteFile to allow it to run in the playground
+ 2021-01-22 eb21b31e48 runtime: define dummy msanmove
+ 2021-01-22 3a778ff50f runtime: check for g0 stack last in signal handler
+ 2021-01-22 a2cef9b544 cmd/go: don't lookup the path for CC when invoking cgo

Change-Id: I651949f9eb18b57e3c996c4f3b2b3bf458bc5d97

test/fixedbugs/issue43835.go

@@ -0,0 +1,45 @@
+// run
+
+// Copyright 2021 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
+
+func main() {
+	if f() {
+		panic("FAIL")
+	}
+	if bad, _ := g(); bad {
+		panic("FAIL")
+	}
+	if bad, _ := h(); bad {
+		panic("FAIL")
+	}
+}
+
+func f() (bad bool) {
+	defer func() {
+		recover()
+	}()
+	var p *int
+	bad, _ = true, *p
+	return
+}
+
+func g() (bool, int) {
+	defer func() {
+		recover()
+	}()
+	var p *int
+	return true, *p
+}
+
+
+func h() (_ bool, _ int) {
+	defer func() {
+		recover()
+	}()
+	var p *int
+	return true, *p
+}