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/subr.go

@@ -1687,7 +1687,7 @@ func genwrapper(rcvr *types.Type, method *types.Field, newnam *types.Sym, iface
 	lineno = autogeneratedPos
 
 	dclcontext = PEXTERN
-	markdcl()
+	markdcl(src.NoPos)
 
 	this := namedfield(".this", rcvr)
 	this.Left.Name.Param.Ntype = this.Right
@@ -1717,7 +1717,7 @@ func genwrapper(rcvr *types.Type, method *types.Field, newnam *types.Sym, iface
 	fn.Func.Nname.Name.Param.Ntype = t
 	fn.Func.Nname.Sym.SetExported(true) // prevent export; see closure.go
 	declare(fn.Func.Nname, PFUNC)
-	funchdr(fn)
+	funchdr(fn, src.NoPos)
 
 	// arg list
 	var args []*Node
@@ -1782,9 +1782,9 @@ func genwrapper(rcvr *types.Type, method *types.Field, newnam *types.Sym, iface
 		dumplist("genwrapper body", fn.Nbody)
 	}
 
-	funcbody(fn)
+	funcbody(fn, src.NoPos)
 	Curfn = fn
-	popdcl()
+	popdcl(src.NoPos)
 	if debug_dclstack != 0 {
 		testdclstack()
 	}