pkg/proc: fix type cast between slices (#4048)

Type casts between slice types with the same element type should be
allowed. In theory this should be allowed by the check on the real type
(which already gets rid of typedefs) but the compiler here actually
creates a distinct type for Message instead of having it as a typedef
for []uint8.

Fixes #4042

pkg/proc/eval.go

@@ -1820,6 +1820,10 @@ func typeCastCompatibleTypes(typ1, typ2 godwarf.Type) bool {
 			}
 			return true
 		}
+	case *godwarf.SliceType:
+		if ttyp2, ok := typ2.(*godwarf.SliceType); ok {
+			return ttyp1.ElemType.String() == ttyp2.ElemType.String()
+		}
 	case *godwarf.ComplexType:
 		if _, ok := typ2.(*godwarf.ComplexType); ok {
 			// size and alignment already checked above