runtime: do not print runtime panic frame at top of user stack

runtime: do not print runtime panic frame at top of user stack

The expected default behavior (no explicit GOTRACEBACK setting)
is for the stack trace to start in user code, eliding unnecessary runtime
frames that led up to the actual trace printing code. The idea was that
the first line number printed was the one that crashed.

For #5832 we added code to show 'panic' frames so that if code panics
and then starts running defers and then we trace from there, the panic
frame can help explain why the code seems to have made a call not
present in the code. But that's only needed for panics between two different
call frames, not the panic at the very top of the stack trace.
Fix the fix to again elide the runtime code at the very top of the stack trace.

Simple panic:

	package main

	func main() {
		var x []int
		println(x[1])
	}

Before this CL:

	panic: runtime error: index out of range

	goroutine 1 [running]:
	panic(0x1056980, 0x1091bf0)
		/Users/rsc/go/src/runtime/panic.go:531 +0x1cf
	main.main()
		/tmp/x.go:5 +0x5

After this CL:

	panic: runtime error: index out of range

	goroutine 1 [running]:
	main.main()
		/tmp/x.go:5 +0x5

Panic inside defer triggered by panic:

	package main

	func main() {
		var x []int
		defer func() {
			println(x[1])
		}()
		println(x[2])
	}

Before this CL:

	panic: runtime error: index out of range
		panic: runtime error: index out of range

	goroutine 1 [running]:
	panic(0x1056aa0, 0x1091bf0)
		/Users/rsc/go/src/runtime/panic.go:531 +0x1cf
	main.main.func1(0x0, 0x0, 0x0)
		/tmp/y.go:6 +0x62
	panic(0x1056aa0, 0x1091bf0)
		/Users/rsc/go/src/runtime/panic.go:489 +0x2cf
	main.main()
		/tmp/y.go:8 +0x59

The middle panic is important: it explains why main.main ended up calling main.main.func1 on a line that looks like a call to println. The top panic is noise.

After this CL:

	panic: runtime error: index out of range
		panic: runtime error: index out of range

	goroutine 1 [running]:
	main.main.func1(0x0, 0x0, 0x0)
		/tmp/y.go:6 +0x62
	panic(0x1056ac0, 0x1091bf0)
		/Users/rsc/go/src/runtime/panic.go:489 +0x2cf
	main.main()
		/tmp/y.go:8 +0x59

Fixes #17901.

Change-Id: Id6d7c76373f7a658a537a39ca32b7dc23e1e76aa
Reviewed-on: https://go-review.googlesource.com/33165
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>