[code-reflection] RFR: Add sample annotation processor using code models

[Maurizio Cimadamore]

This change adds a new sample of an annotation processor that uses code models to:
* reject unsupported methods (e.g. `System.gc`, `System.exit`)
* reject unsupported language construct (e.g. `try`, `throw`)

The annotation processor is somewhat configurable, and can be used by developers as an initial sketch upon which to build more custom compile-time checks.

When using the annotation processor to compile this:


class Foo {
   @CodeReflection
   void test1() {
      System.exit(1);
   }

    @CodeReflection
    void test2() {
        System.gc();
    }

    @CodeReflection
    void test3() {

       try {
           test2();
       } finally {
           test3();
       }

    }
}


The following error messages are generated:


Foo.java:5: error: System.exit not supported in reflectable methods
   void test1() {
        ^
Foo.java:10: error: System.gc not supported in reflectable methods
    void test2() {
         ^
Foo.java:15: error: try/catch statement not supported in reflectable methods
    void test3() {
         ^


When putting this together, I noted some issues, reported below:

* javac was failing to execute annotation processors that depended on `jdk.incubator.code` module
* obtaining the model from a method that contains attribution errors crashed the annotation processor
* some packages (e.g. the ones in java.base) are not exported to the dynamic module layer's `jdk.incubator.code`
* it is not possible to report the message at a more accurate location because the `Messager` API does not allow for this

Many thanks to @lahodaj for the invaluable help when fighting with module layers :-)

-------------

Commit messages:
 - Tweak annotation processor readme guide
 - Tweak documentation
 - Fix test
 - Add test
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/554/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=554&range=00
  Stats: 381 lines in 9 files changed: 370 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/babylon/pull/554.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/554/head:pull/554

PR: https://git.openjdk.org/babylon/pull/554