cmd/compile: output DWARF lexical blocks for local variables

Change compiler and linker to emit DWARF lexical blocks in .debug_info
section when compiling with -N -l.

Version of debug_info is updated from DWARF v2 to DWARF v3 since
version 2 does not allow lexical blocks with discontinuous PC ranges.

Remaining open problems:
- scope information is removed from inlined functions
- variables records 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, even before its declaration.

Updates #6913.
Updates #12899.

Change-Id: Idc6808788512ea20e7e45bcf782453acb416fb49
Reviewed-on: https://go-review.googlesource.com/40095
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -351,12 +351,21 @@ type Param struct {
 
 // Func holds Node fields used only with function-like nodes.
 type Func struct {
-	Shortname  *types.Sym
-	Enter      Nodes // for example, allocate and initialize memory for escaping parameters
-	Exit       Nodes
-	Cvars      Nodes   // closure params
-	Dcl        []*Node // autodcl for this func/closure
-	Inldcl     Nodes   // copy of dcl for use in inlining
+	Shortname *types.Sym
+	Enter     Nodes // for example, allocate and initialize memory for escaping parameters
+	Exit      Nodes
+	Cvars     Nodes   // closure params
+	Dcl       []*Node // autodcl for this func/closure
+	Inldcl    Nodes   // copy of dcl for use in inlining
+
+	// Parents records the parent scope of each scope within a
+	// function. The root scope (0) has no parent, so the i'th
+	// scope's parent is stored at Parents[i-1].
+	Parents []ScopeID
+
+	// Marks records scope boundary changes.
+	Marks []Mark
+
 	Closgen    int
 	Outerfunc  *Node // outer function (for closure)
 	FieldTrack map[*types.Sym]struct{}
@@ -380,6 +389,19 @@ type Func struct {
 	flags bitset8
 }
 
+// A Mark represents a scope boundary.
+type Mark struct {
+	// Pos is the position of the token that marks the scope
+	// change.
+	Pos src.XPos
+
+	// Scope identifies the innermost scope to the right of Pos.
+	Scope ScopeID
+}
+
+// A ScopeID represents a lexical scope within a function.
+type ScopeID int32
+
 const (
 	funcDupok         = 1 << iota // duplicate definitions ok
 	funcWrapper                   // is method wrapper