cmd/compile,runtime: redo mid-stack inlining tracebacks

Work involved in getting a stack trace is divided between
runtime.Callers and runtime.CallersFrames.

Before this CL, runtime.Callers returns a pc per runtime frame.
runtime.CallersFrames is responsible for expanding a runtime frame
into potentially multiple user frames.

After this CL, runtime.Callers returns a pc per user frame.
runtime.CallersFrames just maps those to user frame info.

Entries in the result of runtime.Callers are now pcs
of the calls (or of the inline marks), not of the instruction
just after the call.

Fixes #29007
Fixes #28640
Update #26320

Change-Id: I1c9567596ff73dc73271311005097a9188c3406f
Reviewed-on: https://go-review.googlesource.com/c/152537
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/obj/link.go

@@ -393,11 +393,12 @@ type LSym struct {
 
 // A FuncInfo contains extra fields for STEXT symbols.
 type FuncInfo struct {
-	Args   int32
-	Locals int32
-	Text   *Prog
-	Autom  []*Auto
-	Pcln   Pcln
+	Args     int32
+	Locals   int32
+	Text     *Prog
+	Autom    []*Auto
+	Pcln     Pcln
+	InlMarks []InlMark
 
 	dwarfInfoSym   *LSym
 	dwarfLocSym    *LSym
@@ -411,6 +412,23 @@ type FuncInfo struct {
 	StackObjects *LSym
 }
 
+type InlMark struct {
+	// When unwinding from an instruction in an inlined body, mark
+	// where we should unwind to.
+	// id records the global inlining id of the inlined body.
+	// p records the location of an instruction in the parent (inliner) frame.
+	p  *Prog
+	id int32
+}
+
+// Mark p as the instruction to set as the pc when
+// "unwinding" the inlining global frame id. Usually it should be
+// instruction with a file:line at the callsite, and occur
+// just before the body of the inlined function.
+func (fi *FuncInfo) AddInlMark(p *Prog, id int32) {
+	fi.InlMarks = append(fi.InlMarks, InlMark{p: p, id: id})
+}
+
 //go:generate stringer -type ABI
 
 // ABI is the calling convention of a text symbol.