cmd/link: create DWARF types for autos based R_USETYPE relocs

Switch the linker over to use dummy R_USETYPE relocations on DWARF
subprogram DIEs as a means of insuring that DWARF types are created
for types of autotmp values used in live functions.

This change is part of a series intended to clean up handling of
autotmp types and remove use of autom's in the compiler and linker.

Updates #34554.

Change-Id: Ic74da6bd723ab7e4d8a16ad46e23228650d4b525
Reviewed-on: https://go-review.googlesource.com/c/go/+/197498
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

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

@@ -1766,6 +1766,24 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 	// Create DIEs for global variables and the types they use.
 	genasmsym(ctxt, defdwsymb)
 
+	// Create DIEs for variable types indirectly referenced by function
+	// autos (which may not appear directly as param/var DIEs).
+	for _, lib := range ctxt.Library {
+		for _, unit := range lib.Units {
+			lists := [][]*sym.Symbol{unit.AbsFnDIEs, unit.FuncDIEs}
+			for _, list := range lists {
+				for _, s := range list {
+					for i := 0; i < len(s.R); i++ {
+						r := &s.R[i]
+						if r.Type == objabi.R_USETYPE {
+							defgotype(ctxt, r.Sym)
+						}
+					}
+				}
+			}
+		}
+	}
+
 	synthesizestringtypes(ctxt, dwtypes.Child)
 	synthesizeslicetypes(ctxt, dwtypes.Child)
 	synthesizemaptypes(ctxt, dwtypes.Child)

src/cmd/link/internal/objfile/objfile.go

@@ -342,6 +342,10 @@ overwrite:
 			}
 		}
 
+		// Temporary: zero out the autom list after we've read it.
+		// In a subsequent patch we'll remove autom handling more completely.
+		pc.Autom = nil
+
 		pc.Pcsp.P = r.readData()
 		pc.Pcfile.P = r.readData()
 		pc.Pcline.P = r.readData()