go/internal/gccgoimporter: revise previous anonymous field fix.

Revise the fix for #31540 to take into account the possibility that we
may see an alias to a name that has already been installed into
package scope. This scenario is not possible to reproduce with the
current importer unit tests; changes to the harness to enable this
scenario will be added in a later CL.

Updates #31540.

Change-Id: Ie155d5e0b998604177a78471cba2413f57d40229
Reviewed-on: https://go-review.googlesource.com/c/go/+/173440Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/internal/gccgoimporter/parser.go

@@ -493,6 +493,7 @@ func (p *parser) parseNamedType(nlist []int) types.Type {
 	// type alias
 	if p.tok == '=' {
 		p.next()
+		p.aliases[nlist[len(nlist)-1]] = name
 		if obj != nil {
 			// use the previously imported (canonical) type
 			t := obj.Type()
@@ -502,7 +503,6 @@ func (p *parser) parseNamedType(nlist []int) types.Type {
 		}
 		t := p.parseType(pkg, nlist...)
 		obj = types.NewTypeName(token.NoPos, pkg, name, t)
-		p.aliases[nlist[len(nlist)-1]] = name
 		scope.Insert(obj)
 		return t
 	}