Summary
-------

Add a new lint category `proprietary` that controls the mandatory warning "Foo is internal proprietary API and may be removed in a future release".

Problem
-------

There is currently no way to suppress this warning. This puts awkward constraints on the build process of any affected project.

For example, building with `-Werror` is not possible if the project uses e.g. `sun.misc.Unsafe`.

Solution
--------

Put this warning under the control of the Lint mechanism, so it can be controlled via the `-Xlint` flag and `@SuppressWarnings`.

Note that this warning is currently a mandatory warning, so we will preserve that behavior. However, the warning will be newly suppressible, so that means we will also do the normal mandatory warning aggregation logic, in which we emit a "recompile" note, refer to additional files as needed, etc.

Specification
-------------

The usual mandatory warning behavior will apply, for example:
```
$ cat Proprietary.java 
class Proprietary {
    sun.misc.Unsafe x;
}
$ javac -d classes Proprietary.java 
Proprietary.java:2: warning: [proprietary] Unsafe is internal proprietary API and may be removed in a future release
    sun.misc.Unsafe x;
            ^
1 warning
$ javac -d classes -Xlint:-proprietary Proprietary.java 
Note: Proprietary.java uses an internal proprietary API that may be removed in a future release
Note: Recompile with -Xlint:proprietary for details.
$ javac -d classes -Xlint:-proprietary Proprietary.java Proprietary2.java
Note: Some input files use an internal proprietary API that may be removed in a future release
Note: Recompile with -Xlint:proprietary for details.
```

The new lint category will be added to the `--help-lint` output:

```
--- x0	2025-02-14 15:18:27.887776902 -0600
+++ -	2025-02-14 15:18:40.186611000 -0600
@@ -23,6 +23,7 @@
     overrides            Warn about issues regarding method overrides.
     path                 Warn about invalid path elements on the command line.
     processing           Warn about issues regarding annotation processing.
+    proprietary          Warn about use of internal proprietary APIs.
     rawtypes             Warn about use of raw types.
     removal              Warn about use of API that has been marked for removal.
     requires-automatic   Warn about use of automatic modules in the requires clauses.
```