if first function in file was dead code, it was being
discarded along with the file name information for that file.
leave the functions in the master function list longer:
let xfol take the dead code out of the code list,
and let span skip the unreachable functions during output.

before
	throw: sys·mapaccess1: key not in map

	panic PC=0x2e7b20
	throw+0x33 /Users/rsc/go/src/pkg/runtime/runtime.c:65
		throw(0x5834f, 0x0)
	sys·mapaccess1+0x73 /Users/rsc/go/src/pkg/runtime/hashmap.c:769
		sys·mapaccess1(0x2b9bd0, 0x0)
	gob·*Encoder·Encode+0x16b /Users/rsc/go/src/pkg/fmt/print.go:2926
		gob·*Encoder·Encode(0x2bb440, 0x0, 0x558b0, 0x0, 0x2e4be0, ...)
	main·walk+0x331 :1603
		main·walk(0x33a480, 0x0)
	main·walk+0x271 :1596
		main·walk(0x300640, 0x0)
	main·walk+0x271 :1596
		main·walk(0x300520, 0x0)
	main·walk+0x271 :1596
		main·walk(0x300240, 0x0)
	main·walk+0x271 :1596
		main·walk(0x678f8, 0x0)
	main·main+0x22 :1610
		main·main()

after
	throw: sys·mapaccess1: key not in map

	panic PC=0x2e7b20
	throw+0x33 /Users/rsc/go/src/pkg/runtime/runtime.c:65
		throw(0x5834f, 0x0)
	sys·mapaccess1+0x73 /Users/rsc/go/src/pkg/runtime/hashmap.c:769
		sys·mapaccess1(0x2b9bd0, 0x0)
	gob·*Encoder·Encode+0x16b /Users/rsc/go/src/pkg/gob/encoder.go:319
		gob·*Encoder·Encode(0x2bb3c0, 0x0, 0x558b0, 0x0, 0x2e4be0, ...)
	main·walk+0x331 /Users/rsc/dir.go:121
		main·walk(0x2f6ab0, 0x0)
	main·walk+0x271 /Users/rsc/dir.go:114
		main·walk(0x301640, 0x0)
	main·walk+0x271 /Users/rsc/dir.go:114
		main·walk(0x301520, 0x0)
	main·walk+0x271 /Users/rsc/dir.go:114
		main·walk(0x301240, 0x0)
	main·walk+0x271 /Users/rsc/dir.go:114
		main·walk(0x678f8, 0x0)
	main·main+0x22 /Users/rsc/dir.go:128
		main·main()
	mainstart+0xe /Users/rsc/go/src/pkg/runtime/amd64/asm.s:55
		mainstart()
	goexit /Users/rsc/go/src/pkg/runtime/proc.c:133
		goexit()

R=r
DELTA=46  (20 added, 25 deleted, 1 changed)
OCL=34094
CL=34103