fix line numbers, again.

fix line numbers, again.

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