RFR: Add checks to reject creation of non-empty null-restricted arrays w/o initializer

[Maurizio Cimadamore]

This PR adds checks in `Attr` so that creation of non-empty null-restricted arrays w/o an initializer is not allowed.

The code is very careful in determining situations where null-restrictions on the array element type are irrelevant, such as:

* when the array size is a constant zero - `new String![0]` -- as in this case the restricted array is empty, so there's nothing to initialize
* when one or more dimensions are missing - `new String![10][]` -- as in this case the outer array (of size 10) is just filled with `null`s, as the inner array lacks a size
* when one or more intermediate dimensions is the constant zero - `new String[0][10]` -- as in this case nothing is really created, as the outer array is empty

A test has been added to (hopefully) cover all such pesky cases.

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Commit messages:
 - Fix regression tests
 - Merge branch 'bworld' into restricted_array_new_attr
 - Initial push
 - Remove spurious changes
 - Revert changes to stripMetadata & friends
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1876/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1876&range=00
  Stats: 56 lines in 6 files changed: 53 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1876.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1876/head:pull/1876

PR: https://git.openjdk.org/valhalla/pull/1876