cmd/compile: allow fieldtrack of unexported fields

The fieldtrack support is experimental and used mainly inside Google,
where we have included this change for years. No reason not to make
it in the public copy.

Change-Id: I5233e4e775ccce60a17098c007aed8c82a0425d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/387355
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

src/cmd/compile/internal/walk/expr.go

@@ -1012,9 +1012,6 @@ func usefield(n *ir.SelectorExpr) {
 	if outer.Sym() == nil {
 		base.Errorf("tracked field must be in named struct type")
 	}
-	if !types.IsExported(field.Sym.Name) {
-		base.Errorf("tracked field must be exported (upper case)")
-	}
 
 	sym := reflectdata.TrackSym(outer, field)
 	if ir.CurFunc.FieldTrack == nil {