Changes

Usage warnings can now be disabled label by label. (#11864)

This commit ensures that we read the attributes attached to fields,
constructors or extension constructors before deciding if we should
track their uses or not. For instance, the code below disables usage warnings
for the constructor "B" or the field "b":
```
type s = A | B [@warning "-unused-constructor"] | C
type r = { a: int; b:int [@warning "-unused-field"]; c:int }
type t += A | B [@warning "-unused-extension"] | C
```
 (commit: 8712acc)