RFR: 8331051: Add an `@since` checker test for `java.base` module [v9]

[Nizar Benalla]

> This checker checks the values of the `@since` tag found in the documentation comment for an element against the release in which the element first appeared.
> 
> Real since value of an API element is computed as the oldest release in which the given API element was introduced. That is:
> - for modules, classes and interfaces, the release in which the element with the given qualified name was introduced
> - for constructors, the release in which the constructor with the given VM descriptor was introduced
> - for methods and fields, the release in which the given method or field with the given VM descriptor became a member of its enclosing class or interface, whether direct or inherited
> 
> Effective since value of an API element is computed as follows:
> - if the given element has a `@since` tag in its javadoc, it is used
> - in all other cases, return the effective since value of the enclosing element
> 
> The since checker verifies that for every API element, the real since value and the effective since value are the same, and reports an error if they are not.
> 
> Preview method are handled as per JEP 12, if `@PreviewFeature` is used consistently going forward then the checker doesn't need to be updated with every release. The checker has explicit knowledge of preview elements that came before `JDK 14` because they weren't marked in a machine understandable way and preview elements that came before `JDK 17` that didn't have `@PreviewFeature`.
> 
> Important note : We only check code written since `JDK 9` as the releases used to determine the expected value of `@since` tags are taken from the historical data built into `javac` which only goes back that far
> 
> The intial comment at the beginning of `SinceChecker.java` holds more information into the program.
> 
> I already have filed issues and fixed some wrong tags like in #18640, #18032, #18030, #18055, #18373, #18954, #18972.

Nizar Benalla has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 17 additional commits since the last revision:

 - Improve checker report messages
 - Merge branch 'master' into source-based-since-checker
 - - removed unused parameter `i`
   - make rest of methods private
   - ".toString" instead of "toString"
   
   Signed-off-by: Nizar Benalla <nizar.benalla at oracle.com>
 - Add a few more legacy methods, needed a few more after changes to the checker
 - checking for null values directly rather than catching NPE
 - Merge remote-tracking branch 'upstream/master' into source-based-since-checker
 - Now only skipping the common methods that every record class will have, and will never need a new `@since`
 - - Not checking elements enclosed within a record, I doubt a record class will change after being created
   - Added a null check as `toString` can return an exception
 - - Added some legacy modules that existed long before preview features (they were incubating)
   - Not checking elements enclosed withing a record
   - Only check if the file is readable using `Files.isReadable`
   - Dropped the use of `Files.exists` and `Files.isDirectory`
   - Use `--add-modules` option now to resolve certain modules
 - mention packages in initial comment
 - ... and 7 more: https://git.openjdk.org/jdk/compare/544d8c3f...5be962b7

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18934/files
  - new: https://git.openjdk.org/jdk/pull/18934/files/85b2d1a5..5be962b7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18934&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18934&range=07-08

  Stats: 77006 lines in 1512 files changed: 56440 ins; 13399 del; 7167 mod
  Patch: https://git.openjdk.org/jdk/pull/18934.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18934/head:pull/18934

PR: https://git.openjdk.org/jdk/pull/18934