cmd/compile: fix bug in DWARF inl handling of unused autos

The DWARF inline info generation hooks weren't properly
handling unused auto vars in certain cases, triggering an assert (now
fixed). Also with this change, introduce a new autom "flavor" to
use for autom entries that are added to insure that a specific
auto type makes it into the linker (this is a follow-on to the fix
for 22941).

Fixes #22962.

Change-Id: I7a2d8caf47f6ca897b12acb6a6de0eb25f5cac8f
Reviewed-on: https://go-review.googlesource.com/81557
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ld/lib.go

@@ -1964,6 +1964,7 @@ func doversion() {
 type SymbolType int8
 
 const (
+	// see also http://9p.io/magic/man2html/1/nm
 	TextSym      SymbolType = 'T'
 	DataSym                 = 'D'
 	BSSSym                  = 'B'
@@ -1972,6 +1973,9 @@ const (
 	FrameSym                = 'm'
 	ParamSym                = 'p'
 	AutoSym                 = 'a'
+
+	// Deleted auto (not a real sym, just placeholder for type)
+	DeletedAutoSym = 'x'
 )
 
 func genasmsym(ctxt *Link, put func(*Link, *sym.Symbol, string, SymbolType, int64, *sym.Symbol)) {
@@ -2096,6 +2100,11 @@ func genasmsym(ctxt *Link, put func(*Link, *sym.Symbol, string, SymbolType, int6
 			continue
 		}
 		for _, a := range s.FuncInfo.Autom {
+			if a.Name == objabi.A_DELETED_AUTO {
+				put(ctxt, nil, "", DeletedAutoSym, 0, a.Gotype)
+				continue
+			}
+
 			// Emit a or p according to actual offset, even if label is wrong.
 			// This avoids negative offsets, which cannot be encoded.
 			if a.Name != objabi.A_AUTO && a.Name != objabi.A_PARAM {