runtime: tell race detector about reflectOffs.lock

Fixes #15832

Change-Id: I6f3f45e3c21edd0e093ecb1d8a067907863478f5
Reviewed-on: https://go-review.googlesource.com/23441
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/reflect/export_test.go

@@ -109,3 +109,7 @@ func IsExported(t Type) bool {
 	n := typ.nameOff(typ.str)
 	return n.isExported()
 }
+
+func ResolveReflectName(s string) {
+	resolveReflectName(newName(s, "", "", false))
+}