cmd/compile: output DWARF lexical blocks for local variables

Change compiler and linker to emit DWARF lexical blocks in debug_info.
Version of debug_info is updated from DWARF v.2 to DWARF v.3 since version 2
does not allow lexical blocks with discontinuous ranges.

Second attempt at https://go-review.googlesource.com/#/c/29591/

Remaining open problems:
- scope information is removed from inlined functions
- variables in debug_info do not have DW_AT_start_scope attributes so a
variable will shadow other variables with the same name as soon as its
containing scope begins, before its declaration.

Updates golang/go#12899, golang/go#6913

Change-Id: I0e260a45b564d14a87b88974eb16c5387cb410a5
Reviewed-on: https://go-review.googlesource.com/36879
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -7,6 +7,7 @@ package gc
 import (
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
+	"cmd/internal/src"
 	"cmd/internal/sys"
 	"fmt"
 	"strings"
@@ -3661,7 +3662,7 @@ func walkprintfunc(n *Node, init *Nodes) *Node {
 
 	oldfn := Curfn
 	Curfn = nil
-	funchdr(fn)
+	funchdr(fn, src.NoPos)
 
 	a = nod(n.Op, nil, nil)
 	a.List.Set(printargs)
@@ -3670,7 +3671,7 @@ func walkprintfunc(n *Node, init *Nodes) *Node {
 
 	fn.Nbody.Set1(a)
 
-	funcbody(fn)
+	funcbody(fn, src.NoPos)
 
 	fn = typecheck(fn, Etop)
 	typecheckslice(fn.Nbody.Slice(), Etop)